When buying flight with AAdvantage points do you still pay $ at the end?

First, 50,000 points is not the same as 50,000 dollars. Most airline companies allow you to buy (or top up) your points for money, and you'll end up paying around $10-20 for 1,000 points, therefore 50,000 points equates to $500-$1000 in money.

Next, to answer your actual question, you need to understand how airline tickets are priced. The cost of the ticket is composed of several items:

• actual cost of the ticket that the airline charges you
• government imposed taxes (e.g. X dollars for each short leg and Y dollars for each long leg)
• airport imposed fees (e.g. Z dollars for each outgoing flight)
• airline imposed surcharges (e.g. fuel surcharge, etc.)

Overall, the actual price of the flight is often a quarter of the total amount you have to pay for your ticket.

Now, when you are using your loyalty points to pay for the ticket, these points can only be applied toward the actual cost of the ticket (i.e. the first bullet point above). You still have to pay money for everything else.

As an example, a couple of years ago, I used loyalty points to pay for a business class ticket from London to Sydney on Virgin Atlantic. I paid 200,000 points plus about over $1000 in money for taxes and fees. On the same trip, I used loyalty points for an internal flight within Australia - I paid 30,000 points plus another about $50 for taxes, fees, etc.

Hopefully, this gives you an idea of how reward tickets (that's what they are called) work.

While Aleks G's answer gives a general idea and may apply to other frequent flyer programs, AAdvantage does not simply charge all fare elements other than "base fare" in cash for awards. Government and airport taxes are charged, but fuel surcharges are not charged for flights operated by American (which all AAnytime awards are) or indeed operated by any airline other than British Airways and Iberia - source. For flights within the United States, AAdvantage award taxes are only $5.60 each way. Perhaps some international AAnytime flights would get "up to $60". (Whopping fees in the $1000 range can happen for AAdvantage awards on British Airways, which would necessarily be MileSAAver awards.)

Other notes on your question:

The AAdvantage currency for awards is called miles, not points. (Points have a role in elite status, not directly relevant here.) AAnytime awards indeed appear to have no fees for changing origin and/or destination without changing the award type, but are still subject to the $150 fee to cancel and redeposit miles (except for Executive Platinum members) - source. Note that changes keeping the same origin, destination, and award type are free for all AAdvantage awards.

As noted in Aleks G's answer, 50,000 miles should not be described as "$50,000 worth". An estimate of the effective value of those miles is ~$900. You might, though, earn the 50,000 miles by spending $50,000 on merchandise on the credit card.