Anecdotally, I'd pay essentially up to what I'd fork out for a Macbook Pro now. It just needs to work, the way Apple stuff works, without any headaches. The problem with Apple machines for me now is that they've done all these silly reductions in features that actively hurt my convenient use of it.
Edit, to clarify, I'm talking about a legit copy of OSX purchased properly, no piracy whatsoever. I just want the creature comforts of OSX on a machine with ports! (and MagSafe too, if possible)