I have a Sony Ericcsson T303, and was just looking on ebay to get an idea of how much they sell for...
Brand new they are going for up to £90 on buy it now, and used have been bidded up to £40 with a lot of time left...
However, I looked on the Carphone Warehouse site, and found them for as little as £29 brand new, up to £40, depending on the network you want.

Ive noticed that mostly, if the phones are locked to a particular network, the price can be up to £50 cheaper than unlocked ones.
Since unlocking generally costs £5-10 in a local mobile phone shop or a market stall, its not worth payin that for an unlocked phone.

Its a good little phone by the way, pretty basic, but not bad if you dont want too many features, camera isn't up to much, but its a nice looking little thing...Only 10mb internal memory and no card slot, but a good, solid, basic phone, with a few nice extra features.
Just don't go paying too much for it ;)

Guide written: February 2009