I would say it was because you are looking at Retail prices from vendors. Are little n900 now has a quite a reputation for itself and is also in short(ish) supply so they may have upped the price for new units since?

You should keep an eye out on the buy&sell pages of the forum or check out ebay, a very well kept n900 can often be had for as much as £150.

This guy has one listed for £79 with 10hrs left and a BIN price of £149 if you wanna take a punt