Not too long ago I purchased a couple of NGC from CeX luckily the prices were low whereas now they have boosted them a bit too much to the point of being ridiculous. The Legend of Zelda: Twilight Princess was £32 now £48, and to top it it's a used copy not even new. Melee was £12 now £20 and so forth. Good thing I didn't wait for the prices to drop / Christmas season to end because I'd be screwed.
Nintendo games do generally hold a really high price, however i do agree £48 is a bit of a joke. The best time to buy them new is when shops are selling them off usually after a new console release or the game is been sold off. I got Meelee for about £30-40 brand new 10 years ago, i think given its still worth £20 now second hand its depreciated really well!