Recently I've noticed that a lot have drastically dropped to really cheap, though, but out of print / popular games / still somewhat recent haven't. Nintendo Wii quality games haven't dropped but it's not news to anyone since they hardly ever do (Super Smash Bros. 64 cartridge alone can cost £30!), although now most of them cost around £10-£15 which is great.
In general used games on the Xbox 360 have always been pretty cheap i think, what price point do you call cheap? Ive got a fair few games for around £5, Modern Warfare i got for £5 and Unreal 3 for £4. In the past ive got a fair few games in the £5-10 price bracket.
Dropping down to £4 more or less, though these kind of deals usually are put up by Zoverstocks which offer the lowest price of all sellers. The kind of games they don't sell are NES, SNES, N64 and the best of DS, but if they do then they're snatched quickly.