You don't
need two with a 12x12 room, but for $250 I wouldn't go with two subs, I'd go with one. Yes it is a bit low, but you also don't need a monster sub for a room that size. The thing working against you is the 12x12 aspect, as a squarish room has the most cancellation; two might actually help in that instance, but not necessarily with a budget like that. I might look at the Sub120 from
www.partsexpress.com to start with one, and it is currently $155.