you can get cheap home powered subs, keep looking.
you would use your current left/right speaks
the circle thing with a prong is most probably an rca cable, the standard hook up
if its not an rca cable, you can just run your left/right speaker leads into the sub(called high level, or line level input)
you need to figure out if your prong thing is indeed an rca cable, if not you need a sub with high level inputs
a powered 10 will shake your house, a 12 will rattle stuff off walls. my sony 12 was 250wrms and was $200
edit: heres a powered 10 with 100wrms(225w peak) for $100
http://cgi.ebay.com/JBL-Balboa-10-10...3A1%7C294%3A50