Bought mine for £10k and if you want a keeper for that kind of money then you are going to spend almost the same again to get it up to standard. Or just buy it, drive it for a few years and break it for parts worth almost double what you paid. That said, with all cars of that age even the "mint" ones aren't and need it all doing. I'd recommend a Z4M to anyone now I've got one, just get ready to start emptying your bank account.
Z Coupe models are generally more expensive than the equivalent roadster, and the Z3's coupes worth more. I'm not sure why the Z4 Coupes don't make the big money, but the classic car market (which the Z4 hasn't reached yet) is a fickle market.