They used to be super cheap - like 200 - 300, but now they range quite a bit based on condition and if they still have the original hard case - I think it is roughly like 400 - 650. Still a good deal, in my opinion.
I paid $325 for the one I got in fair condition last week. It's a little more than what an average T-40 should go for in the same condition, but it was a rather rare early '78 with the original rectangular case. The one I got the week before was $800, but it's another (less) rare early '78 in an uncommon color for the year and in *extremely* good condition, and again with the original case (also in great condition).