I recently bought a Trust TR-16529 tablet which has got a 250x150mm working space for £50 from Currys. It works perfectly well and the only thing that I would like to be able to alter is its movement sensitivity when working in non relative mode.
I'm fairly sure that it was end of line though, so you might have to hunt for one if you are interested; the equivalent Wacom would be about £150.
Edit - I should have said that I'm working with PS rather than Gimp.