What's the cheapest way to travel from Madrid to Barcelona?

The normal train is, as described by santiago, still available. You can check the tickets in www.renfe.com. They are not very comfortable, but at least you can walk around and, eventually, spend some time in the bar. The travel time on these regional trains is around 8 h and price is around 50 euros.

You can also get a fast train, AVE, that takes around 3 h. The prices are quite higher in this case, but you may get lower prices if you buy in advance. ALVIA is similar, only that a bit cheaper and slower.

I would also recommend you to check the buses. ALSA is the main company and has a lot of connections every day. The price is around 32 euros and it takes almost 8 h. They normally stop after 4 hours around Zaragoza in a rest area where you can go to the toilet, get some food...

In order to compare, I did try to buy a single train and bus ticket from Barcelona to Madrid for next Sunday 6th of June.

Based on this, I would suggest you to adapt to what matches:

• If you don't want to care about the quality of the transport and get a constant price no matter when you buy the ticket and how much in advance, go for the bus.
• If you can travel on strange, bad hours, and get the ticket in advance, go and get an AVE.

I can't believe that no one has mentioned ride-share sites yet. blablacar, for example, is hugely popular in Spain and many other European countries. From the looks of it, you'll have no trouble finding a ride along that route and the going rate seems to be ~€30. Driving time is about six hours according to Google Maps.

According to the Eurail site How to get from Madrid to Barcelona by train there are indeed regional trains between Madrid and Barcelona, where they state that

Taking a regional train from Madrid to Barcelona means more travel time than by high-speed train. It also means you don't have to reserve a seat and you can stop along the way in charming Spanish cities like Zaragoza and Tarragona.

and in terms of cost, an example of the types of ticket that you could use on regional trains:

If you have a Eurail pass, you don't need to buy a ticket. You can travel on this train for free and get on and off whenever you like.

Timetables can be found here, for the regional trains, select the "Avoid trains that require reservations" box.

They do have a Eurail Spain pass.

Also, of help is the website Train travel in Spain: train types, other tickets (other than the example above), fares, discounts & other useful info

Here an up-to-date answer 3 years after the question, just for further reference.

I liked how fedorqui compared it, so I'll follow the same approach. I am surprised that flights were rarely mentioned, they can get very cheap sometimes!

The search is done for the 17th of August 2018, about 1 month in advance:

Additional tips

• There are some buses departing at night and arriving in the morning. If you don't mind sleeping in a bus/coach, it is interesting because you get a good price and you don't waste time, as you are sleeping.

• For flights, one should consider also the time and price to get from the city to the airport and from the airport to the city.

• If you are traveling in a group of 4, Renfe has a special group ticket called "Tarifa Mesa" and you can get some discounts. Otherwise, you could also check in some Facebook groups and join others for such a group ticket.

• Renfe is planning to launch a low-cost sister company in 2019 that will be called EVA and is supposed to be 25% cheaper. More info here

• If I remember well, ALVIA services were suspended for this route.

• Due to the wide choice there is and the variability of prices, it is recommended to use a multimodal travel search engine to compare all options for an specific date. Travelinho is well suited for such cases. Disclaimer: as one can assume by the name, it is an own created website.

The absolute cheapest way is almost always by coach, if you're booking in advance. Looking on GoEuro and similar sites, it suggests you can get a coach (7h50) today at 2pm for £6/€8.50 - that's last minute, so the other prices for coaches today aren't dissimilar to trains, but as mentioned, if you're booking in advance, I'd expect that to be the price.

Looking on the Alsa site confirms that's the right ballpark. Buses today and this week are mostly €32, but if you're more than 5 days out, they drop to €15 and then again. So €15 is emininently possible, and €9 a possibility, but that looks like a limited promotional fare.

According to Loco2.com, you can pay 32 GBP (45 EUR) to get from Madrid to Barcelona on the slow, regional train in nine hours. However, you can also catch an early morning (6:20am) train that'll get you there in three hours for 37.50 GBP (53 EUR), which might be worth the extra convenience.

It's not necessarily much more expensive to fly if you do it at the right time and book well in advance. By Vueling, for example, you can fly from Madrid (MAD) to Barcelona (BCN) on Tuesday 9 June at either 10:00 or 12:10 for 45 €, including all fees and 10 kg hand luggage, if you pay by debit card. Checked luggage is 13 € extra. The flight takes only 1 hour and 15 minutes, but you do have to arrange (and pay for) transport to and from the airport.