From the Renfe website, it would appear that if you take the overnight "Tren Hotel" on a tourist (Turista) class ticket, then the cost (Madrid-Barcelona) with the cheapest type of bed is 9,700 ptas. This consists of 6,500 ptas for the ticket, and a supplement of 3,200 ptas for the bed. This is for a single journey.
[This message has been edited by El Boqueron (edited 10-19-2000).]