Regulators are one of the best pieces of gear to buy used.
It's pretty hard to screw up with one, and they usually clean up nicely even if it hasn't been treated in the best manner.
I got 5 of my 8 Atomics used, 3 were in great condition, and 2 were pretty ugly inside (salt water inside the 1st stage for months) but cleaned up to be 100% functional. Ti2 second stages are impervious to corrosion, so they are pretty much guaranteed to be like new.
On Ebay, you can buy used B1's(no octo) for a bit under $300, and Z1's for a little less.
As Charlie99 said, make sure you get one with the Jet piston. (On the ones with the Jet piston, the rubber trim part on the first stage mostly covers the HP cap instead of being donut shaped.) If it's not, the upgrade will cost you about $70 in parts IIRC.
You may have to wait a little to find one.
Atomic Ti2 octos cost quite a bit, usually about $200 used. (The second stage is 2/3 the price of a first+second stage set, with the first stage going for about $100.)