I'm sure you can find a good sub at or below $500. Successfully integrating a single sub by ear is another matter. I've spent a good portion of the last two years fine tuning subs in my system and, IMHO, a sub controller is virtually essential. A stand alone device (like my Velodyne SMS) will start at app. $450, then add the cost of a sub. You could also use an HT receiver with Audyssey which might get you to the same place for a bit less money, but I don't know if that would appeal to many folks on this site.
Marty
Marty