Other than DIY, $500 options include the SVS PB-1000 and Monolith THX Select 10, both 10" subs. The next one up the price ladder is the Rythmik LV12r for $549. I would personally go with the Monolith 10, which is THX rated for a 2000 ft^3 room, which your room falls under. I just recently picked up the Monolith 10 for $425 through one of the ebay coupon code sales. If you can wait for another one of these sales, ebay seem to have them at least once a month. They even had one a month or so ago that was 20% off up to $100, so the Monolith 10 was $400.
Subs>RBH I-12e, I-12. HSU VTF-15H mk2. Monolith THX 12(x2)/THX 10(x2). XTZ 1X12. SVS PB-1000(x2). Speakers>JBL Studio 270/235c/230/225c, Arena 130/125c. Sony CS3/CS8/CS5. QA 3020i/3090Ci, 2020i/2000c. Monolith Cinema 5bs. Polk RTiA1/CSiA4. Other Audio>Sony MDR-Z7m2, MDR-V6, XB900n, XB700, XB32, XB31. Sennheiser HD58X, HD4.50, PC37X. Philips SHP9500. SIVGA SV004. HyperX Cloud PS4. TB Stealth300. LG FH6,RK8,RK7,PK5. HK Onyx Studio4(x2).