IMO subs are important. I would not buy a $250.00 sub (unless it was a used good one ;p). I would save up until I could afford a good sub or two. If you buy a low end sub, I'm pretty sure you will get what you pay for and regret it down the road. I also think you should call svs, hsu etc.. and see what they would recommend for your room. That way you will know about how big/powerful of a sub you will need in that size room. That or you could build your own sub...it would be a lot cheaper; however, I don't know how much time you want to invest in it. There are many audiophiles on here that could walk you through the process. Thats probably your best chance at getting a good sub at a cheap price. You could go with a couple of Daytons....a lot of guys on here recommend that. Just dont settle for something because you will be unhappy in the end.
Good luck and keep us posted!