^^^^ Try looking anywhere
apart from a specialist game shop.
This means places like WHSmith, Asda, Tesco, Woolworths etc etc
You see, the specialist game shops know that people will still buy GC games as there is a back-catalogue that Wii owners can still use. However, as far as the "average non-gaming outlet" is concerned, the Gamecube is old news.....gone the way of the Dodo.....so as far as they know, they just need to shift stock on an obselete console.
Hence, gaming places will keep prices high, non-gaming places will sell it off cheaply.
I myself bought a shed-load of new, sealed GC games from Asda last summer as they desperately tried to shift what they obviously considered "old stuff".
Got things like Zelda/F-zeroGX/Metroid Prime etc etc for about £5-7 each.
.....so keep an eye out in the "other" retail outlets

too, as you might grab a bargain.