Well, so far, I've spent $650 on a set of used regs, another $270 or so getting them serviced, and $480 on a set of small doubles and a doubles wing. It would have been about twice that if I had bought new, and the doubles I bought are 72s and good for training and my normal diving, but likely wouldn't suffice for decompression even for me. And it seems pretty clear that, if I were doing a lot of doubles diving, I'd want at least two sets, because you can't easily walk in and just ask somebody to fill your doubles while you wait -- and if you're sometimes doing mix dives and sometimes not, you don't want to use expensive gas on shallower dives. And then there are deco bottles and their regs . . . It's clear the costs just escalate as you go on.
There was a thread a while back on "What does it cost to do a 300 foot dive?" that you might find illuminating.