Budget Subwoofer

Here is my (somewhat mismatched, but I've had the fronts since '85) system:
Denon avr3801
Polk CS400i center
EPI 1100 lr
Polk M3 rears

Any suggestions for an inexpensive sub - something I could pick up used in the $200 range until I can afford something better?

Thanks for your help - Happy New Year!
Is the sub for movie? If it is, then a visit to Abt Electronic will give you some ideas re. the $200 range subs. If it is for music, then experience tells me that first I need to match my sub with my main speakers perfect, otherwise, you will be unhappy. And to match them well, unfortunately, $200 sub, even used, might not be good enough. My suggestion is: know what speakers will work with what sub in the high-end category, and save enough for that package. By the way, many audiophiles will look for main speakers without sub, so changing your main speakers can also solve your problem/need. Just a thought.
A good sub at $200 used will be hard to come by. Why force yourself to go that route? I sincerely suggest you save some more before you consider one, even used. $500 will get you something good, used.