Renfe Tickets

They certainly don't make it easy and intuitive.

Elige (almost) always has two prices. 'Elige' means choose—lower cost for 2nd class seats, higher for 1st. This ticket category also has 'mid-grade' flexibility, regardless of the seat.

I don't see multiple prices for Basic or Premium on those dates.

Basic is 2nd class seats- 2+2. Most restrictive on flexibility.

Premium is 1st class seats, 1+2, and service: Food and lounge access. Most flexible. (Elige Comfort is the same seats- the same cars, but without the services.)

*edit to add: If Premium isn't available, it means the itinerary doesn't have food service, and you can only buy the bigger seat in the Elige category.

It's very strange. It shows Premium for 117Euros and then the next line, which seems to be the exact same train, shows Premium for 149.65Euros.

I see that now—Those itineraries are listed as different train numbers, which must be some glitch in the system. It's not that way for any other train on the day. It might look like the same thing is happening on the 9:05 departures, but it connects to a different train.

There's a connection? I thought it was direct. So confusing.

Oh, interesting. There are just a few that don't connect, that being one of them. I didn't know that was possible.

Maybe the site is glitchy tonight...I'll wait and try again tomorrow. Thank you for your help!

Looking at bahn.de and Rail Europe shows that train 3931 (the first, cheaper, one) is running that day. Bahn.de shows that 3931 is a 'dated train'- ie one that runs on certain dates only.
Later in the year, outside the date span for 3931, the same train with the same stops becomes 3943 (the second, dearer, one) according to bahn.de.

So it looks like it is a system glitch, and that the first train is the one to book. That's the one which Rail Europe would book for you, if you went through them. 3931 is also the train which 'the trainline' would book for you.

Thank you so much. :)