Pokemon Silver is anything but rare.
On GameCube though? And do they even have any even if they actually list it at that price?
My GS still has a small GCN section and I remember seeing it for that price. I just checked their site and it's around there too http://www.gamestop.com/gc/games/legend-of-zelda-twilight-princess/42893
Granted finding it new is still next to impossible. I don't know how much scalpers or selling them for.