Spainish AVE seems to have little or no advance fares, and time after time on checking, the minimum 4-week advance type fare for the two hour Barcelona-Madrid hop (during daylight) seemed to start at 80. In contrast the airlines doing it start at 19.
Perhaps RENFE's faresetting has not got the right balance. One train I went on some years ago was full. Well, the carriage I was in. The other seven coaches were almost totally empty... couldn't hardly see one head in any of the other carriages. What a mess.
It can be difficult to get cheap fares given the quota once used is used up, but I have just had a look and can see plenty of availability for a random trip in September where each leg is at the 45 mark. Don't forget the site automatically gives you the cheapest for a given train, but it does so irrespective of class/conditions. I always play around with it to find what I want, given there are also different price tiers by ticket type as well (like airline fare buckets). Also as with UK it depends what you want in terms of flexibility, class, etc. Looking at it a promo in turista can be had from around 30. As to seats - like most booking systems it tends to allocate from a given carriage for a given class and they are compulsory of course so that would explain bunching (although now you can pick your seat when you book online).
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To be clear, there are things RENFE can do better but as a product AVE, AVANT etc are generally very good. The same goes for each country and its high speed networks, they all have their quirks and different ways of doing things.
On a couple of things posted:
Re FEVE. As that is a metre gauge network, it is very slow (given most of the network is along the north coast in the mountains) and while now part of RENFE was independent and essentially runs as such. I do agree that the web is pretty light on FEVE information, but it is there (certainly in the relevant Spanish languages e.g. Castilian/castellano and Basque/euskera where EuskoTren runs the service).
On cross-border links to Portugal. They have as others have said always been poor and really only focused on the Madrid-Lisbon route (hanging on in there, but with reduced service). I can see the politics of eventually getting the high speed line finalised, but it is still a way off.
As for other links, I am just not sure there is the demand. While on a rail site it would be nice to see them upgraded and used properly for passengers, the motorway networks have whatever cross border traffic there is including coaches for example between Seville and the Algarve in the south for example.