My guess is as follows:
Last gen, as you remember it, is 4-5 years into the generation.
The games from year 1 and 2 are now 15-20 dollars. Thus, the newer old games also get that price.
It will probably be like that for X360 in a year or two. Ps3 2-3 and for Wii it could take a lot more.
http://www.vgchartz.com/games/userreviewdisp.php?id=261
That is VGChartz LONGEST review. And it's NOT Cute Kitten DS