Spain trip Barcelona and Madrid

Buy it now if it is available. The prices go up as capacity decreases. It is possible for the train at the time you want to sell out.

You can save a lot of money by purchasing an AVE train in advance

Agree, buy ticket now online. We did, and it was considerably less.

* Edited to include we used the renfe website.

I've always bought from the official site, renfe.com, but many folks here have found loco2.com to be more-user friendly and same prices, I believe.

Thanks- Which website is best to buy the tickets? I thought a one way ticket on the AVE from Barcelona to Madrid would be around $115 USD?

I think you'll find both trainline.com and loco2.com are now charging fees, so renfe.com will be cheaper. It is sometimes tricky to use, however.

There is not one, single, price for Barcelona-Madrid AVE tickets. Fares vary over time. When tickets for a specific date initially go on sale, there are some low-priced promo tickets available. As other travelers buy those cheaper tickets, only more expensive tickets remain, so fares trend higher as you get closer to your travel date. I see a lot of trains for Tuesday with fares under 110 euros. I see some 85-euro fares in December.

In general, the longer you wait to buy your ticket on this route, the more you will pay, but you will probably see at least some slight variations in fares for different departure times on your date.

What is your travel date?

Renfe.com doesn't accept many US credit cards, but Paypal has worked well for me and the fees/exchange rate premiums are low.

I bought my tickets in advance, over a month in advance, and they were about 60 euro one way. It saves a lot if you can plan ahead!

You can even find facebook groups in which people resell their tickets at cheaper fares.

Time-saving tip - If a foreignor, do not try to use Renfe app (Android one at least) and instead use the renfe.com web site to purchase tickets. You can leave passport / ID # field blank. Make sure you put the right email address since tickets (PDF attachments) will be sent there.
The Renfe app has issues (even after I got around the phone number limitation by using a dummy number) it would not allow me to buy tickets (cost me a few hours in the middle of the night).

As suggested, buy (AVE) train tickets in advance when possible since peak time trains fill up quickly.
Also buy tickets for popular sights / tours in advance; some sell out quicker than train tickets!