The stock answer, of course, is "whatever someone is prepared to pay for it."
As you may know, I'm a huge Firefly fan. I own the core rulebook and one scenario book for the Serenity RPG, and I'd dearly love the rest.
You can thus imagine my surprise at the "Six Shooters and Spaceships" listing on Amazon. "6 new from £247.19 - 3 used from £157.57".
Bu-wha....?!?!?!?! (as Mal would say).
Even more so when the most I can find one selling for on eBay is £43.
'Verse is plumb kwong-juh duh sometimes, mm?