I recently chanced upon a TMNT for a friend, who wanted one. As a partially working project it cost £1,000 and with the work it needs (Led's, new plastics, new DMD, new sound card and bits and pieces) it will come to just under £2k, which I think is all it's worth in vgc. In hind sight, that TMNT was probably worth half the money I paid for it, but it was bought unseen - and its a bit rubbish, though my mate is happy enough.
Bargains do exist but you have to be very lucky, as others have said, people are clued up on values these days.
My own project was an older machine of mine with a beat up cab, I decided to completely overhaul it - new playfield, ramps, wires, components, cab. Might take me a while. Thats on a working Theatre of Magic I bought 4 years ago for £2k, which would be a bargain now. I ended up led'ing it, adding improvements throughout, colour dmd etc.