As others have said a used shearwater in good condition may manage to fit your price point, but Suunto Zoop will meet your needs for awhile (it's also a very common rental computer so might be able to come well under your price point used...). The thing that really pissed me off on the Zoop was the 24hr lockout if you slightly stray from the computer's extremely conservative settings, not that you should go against what your computer says of course..
The only other computer I've owned is the ScubaPro Mantis M1, which has big pro for me if apnea mode and used might fit your budget (it also has 24 hour lockout, even if simply accidentally had in SCUBA mode when taking a warmup freedive it's likely going to require looking through the directions to be able to reset and use it on that freedive outing...)