In a stationary spot (a desk or the like)?
Get an old , reasonably good, 5.1 AVR from a pawn shop or the like. Should be $30 or so. (you want an AVR to peel off the low-frequencies for your sub, so resist a stereo receiver)
Get a pair of nearfield bookshelves with good off-axis performance. I'd recommend looking at the Behringer 2030p and the Infinity Primus 163. ($200 a pair or so)
Spend what's left on a small sub (Dayton has some decent gear at parts-express.com)