The threshold can't be determined to a particular number, because so much of it depends on your facts and circumstances, and what you show (and tell) CBP.

And it's worse than that. It casts shade on the purpose for your visit, risking a refusal.

What I'm about to say is unfair. For many of our HNQ visitors, it's shocking that the 4th and 5th Amendments don't seem to apply at the immigration desk*. But I am answering OP's question. OP is not asking what fairness ought to be; we know that answer. OP is asking what is likely to happen.

You're a citizen of an advanced country that has a state-of-the-art banking system, competent and uncorrupted law enforcement, and is a member of G8 and all the relevant banking treaties. Not every German travels to the US, only certain ones: and the ones who visit for (your visit purpose here) tend to reside in a certain socioeconomic class. The vast majority of them simply use that banking system in the normal way. Which makes you a "high nail": a standout.

So this raises questions about you. Aside from the question of civil forfeiture, it raises the question of whether you should be admitted in the first place.

You would first, need a darned good reason for wanting to do this, that jibes with your story, believable documents, and allowable purposes for visiting the U.S. And second, have that reason be one that sounds perfectly reasonable to US CBP.

Remember how immigration control works. They make a presumption of immigrant intent: if you refuse to defend yourself, they presume your intent is immigration. That is reasonable, since their mission is controlling immigration by stopping immigrants, and only allowing in people who are short-term travelers. Then, they allow you to present your story and facts (documents) to prove yourself not an immigrant.

Your main weapon in rebutting immigrant intent is to show ties at home. One of the "ties" they like to see is an active, regular banking life. Having a pile of cash suggests the very opposite: a person who is "unbanked"**. Such people tend to overstay their original promise, get in trouble, seek social services or employment, or even resort to crime. Also, they tend to be poorly rooted back home, and that introduces the risk of them immigrating.

So the pile of cash will "light up" questions like this about your visit. Aside from having a reasonable reason for the cash pile, you should be well prepared to document your home ties back in Germany. Owning a home, being politically active, having a well-developed career that doesn't lend itself to telecommuting, that kind of thing.

You'd be sorting all this out with immigration control (CBP) right there at the immigration desk at JFK Airport, and if they refuse you, you'd cross an ocean twice for nothing. Here are two ways to dodge that.

• Don't rely on VWP; apply for a visa, and say right in it, "I know I am VWP eligible, I want a visa because I plan to bring $6000 cash for Very Good Reason Here, and I want to disclose and clear that in advance." (that is not something a criminal does). If they had a problem with that, they would hang up your visa or contact you. Immigration loves this kind of advance disclosure, not least because criminals don't do it.
• Book via Dublin or Shannon. Those airports have CBP Preclearance, which means the CBP desk is right there. If they refuse you, you can grab your bags, leave the airport, pick a four-leaf clover for next time, and head to the nearest pub to drown your sorrows in a 568ml Guinness.

* Because a traveler entering a US border is explicitly not protected from search and seizure. As to the right not to self-incriminate, yes, that applies in full and a visitor can plead the 5th at the immigration desk, and that will stop further questioning. However, she will also be refused entry, since she has no right to enter the US (that's what citizenship means, and certain other limited categories.)

** Unbanked: A concept we are VERY familiar with here in the US, and often taken as a bad sign. Again, within the demographic of ordinary travelers, this either means a very unusual philosophy on money, or (will be presumed to be) inability to bank, due to a blacklist arising from bouncing checks and not covering it, presumably because of not having the money.

I realize this is an open-ended question, and there is no precise threshhold -- that much depends on the authorities I come in contact with. But can someone give me some idea of where the effective boundary is?

Your question cannot be answered, as the way you've asked it implies. Because there is no precise threshold, and because it depends on specifically which authorities you encounter, there is no effective boundary.

All we can tell you is that if you carry enough cash to have to declare it, and you do declare it, you're guaranteed to encounter CBP. CBP does not share the policy that some municipal police forces have of abusing civil asset forfeiture to acquire resources for the department, so filing the form probably does not increase your risk very much.

If you fail to declare your cash despite exceeding the limit, then there is a chance you won't encounter any authorities over the cash, but if you do encounter CBP in a spot check, you're pretty much guaranteed to lose the cash.

There is no 'safe' limit.

The amount you can safely carry is the amount that you are prepared to lose.

First off, let's differentiate the two Civil Asset Forfeitures(CAFs):

• Police - This tends to be focused on people committing (or merely suspected of committing) smaller crimes (i.e you had illegal drugs on you) and having assets seized. The US Supreme Court recently limited actions like this, but they still occur with some frequency. Most of the CAF examples you find via searches are of this type.
• CBP - These tend to be aimed at people believed to be actively doing something like smuggling, laundering money, etc.

The CBP doesn't seem to do this as often, but that's probably because there's a lot more police and people in the general population than go through customs. The largest customs case of CAF centers around an Albanian man who tried to fly with undeclared cash. CBP miscounted the cash and then seized it under a variety of laws aimed at money laundering.

But can someone give me some idea of where the effective boundary is?

The main thing is you don't need to stand out to where they decide to inquire about why you have a lot of cash. CBP might just check your documents and let you through. If you like to smoke marijuana or do drugs, I would make sure I refrained from that for some time before and during your trip, lest a CBP dog indicate you, or your baggage, smell like drugs.

I would also get a visa. You might not need one for your visit, but it's far less likely that someone who has gone through that hassle for a simple visit is trying to launder money.

I know you're aware, but it bears repeating that if you want to bring more than $10,000 in cash, be sure to declare it because the CBP website directly states

Failure to declare currency in amounts more than $10,000 can result in its seizure.

There's a large chance they will use CAF to seize it if you're caught like this.

You didn't give many details, so I'll make some assumptions:

• You're carrying less than 10,000 USD. (The threshold for filing FinCEN 105.)

• There is no other reason the US government would be suspicious of you. (You have a suitable reason for your trip, and you have no immigration violations, criminal record, involvement with drugs, etc.)

• If asked, you would answer truthfully how much money you're carrying.

• If asked, you would give a good explanation of what you plan to do with the money. (Such as paying for food, taxis, and/or some extra in case of unforeseen needs. Bear in mind that US hotels aren't always happy to take cash...at least they will want a credit card on file.)

• If asked, you would give a good explanation of where you got your money. (Such as from your own job, or from your family member who earned it at their job.)

• You have a bank account and you're carrying a credit card or debit card, even if you prefer not to use it.

• Losing all the cash you're carrying would be aggravating, maybe even make you angry enough to avoid the US forever after, but wouldn't really be a major life disaster. It wouldn't change your family situation, career, health, etc.

Based on the above assumptions, you are definitely too worried about this. Millions of tourists visit the US every year with varying amounts of cash, and the amount lost to civil forfeiture is minuscule in comparison. Civil forfeiture laws are a disgraceful abrogation of civil liberty, but their application is rare enough that it is unlikely to affect you as an individual who satisfies the above assumptions.

EDIT: what I did get was a range between $300 and $10000. I was hoping to narrow that down a bit. --- If you satisfy the above assumptions, any amount of cash in this range is fine and will not put you at any significant risk from civil forfeiture. (However, do be careful about regular theft or robbery, which are prevalent in some areas.) You can bring a bit more than you expect to spend, which would depend on the trip length, destination, and level of luxury.

To balance other answers that focus on the risks of carrying cash, I would recommend that you should have a minimum of $300 cash when traveling internationally to the US. This should be enough to pay for a taxi, a few meals, a phone card, and a couple nights in a hotel in most US cities. You don't have to use the cash---you can use your credit card only, or plan on meeting a friend---but you should have it just in case your plane lands in the wrong city and/or your credit card doesn't work in a new country, and you need a day or two to get it sorted out.

There is no such thing as a lower bound. Civil forfeiture laws and practices vary greatly from state to state. See, for example, https://ij.org/report/policing-for-profit/grading-state-federal-civil-forfeiture-laws/

It is certainly a rather unfair and frustrating practice of law enforcement, but on the other hand, it's not common either and the chance of you are encountering such a situation as a tourist a quite small.

The best protection is to carry no or little cash. Credit and debit cards are widely accepted and if you really need cash for anything, you can get it easily at the nearest ATM. Cash is used less and less: For example the food truck outside of Boston South station where you buy your morning coffee doesn't accept cash at all: plastic only. I sometimes go months without using cash.