Scott - I think you're going to be pretty limited there. However, a cheap sub can be fine if you're just looking for a little additional "thump."
Here's the thing I'd keep in mind - Depending on how it's connected or will be connected in the future, there's an important feature - Stereo inputs.
A lot of subs are made to squeeze into home stereos that have a mono sub out. Those are fine if your controller / console / whatever has a mono sub out. Most also have inputs for speaker level, but if you're using active cabs, that isn't going to do you much good.
If you're splitting or going parallel, such as using the "C" switch on a Central Station or Big Knob, you can't just combine the stereo outputs to a single connector - You need to send both signals.
SO - Basically, when you're looking for subs in your price range, keep the stereo input in mind - It'll probably cut out half the units out there right off the bat.
I believe the MTX SW2 would be a nice choice if you still have your CC connections...