Should be 10€ in 2nd class, see this page (scroll down to Spain).

Unfortunately, Belgian railways recently stopped selling tickets to/in Spain. You best chance is to call Deutsche Bahn (+49 30 2970). They can book these trains if you give them the train numbers (3930 and 3931). You can collect them from a DB ticket machine or they can send them via the post for €5.90.

Of course you can also book them at a ticket office in Spain. A few days in advance is usually enough.

Also be aware that Spanish railways (RENFE) don't share their complete timetables with the rest of the world, so every planner is very incomplete for Spain, except for the RENFE website (which is quite clumsy to use) or the Rail Europe website (who managed to get Spanish data somehow).

These days are the end of in ESpana very much used and taken Easter hols. There is just 1 train/day between these (coupled in Cordoba to set from Malaga). You can check now if there are even normal tickets availalbe on renfe.com-best to do it all in ESpanol.

I also doubt now very much that RENFE still allows all those passholders for just 10€ in its expensive AVE on long distance, but you have to find out yourself what the actual price is.

Just curious: did you see any indications anywhere that the 10€ price is not correct anymore?