As a general rule, I'd suggest using loco2 rather than Rail Europe - better site and often cheaper too. For a random day in August, they're showing the overnight bus for €39, or the sleeper for about double that. Capitaine Train are pretty good too, but don't have links with quite as many country's rail booking systems. (Capitaine Train win on supporting loyalty cards, though Loco2 tell me that they're planning to support those too, in the not too distant future)