If you're looking for low-budget speakers for listening to music, I think it's much better idea to put your money to more decent 2.0 speakers and ditch the subwoofer completely, because cheap subs are actually pretty terrible and sound bad. With 2.0 speakers for the same price, you get that much better speakers, and good speakers can usually produce pretty good and more accurate bass sound compared to a cheap sub.
I used to have Creative Gigaworks T40s (DS has them now):

They have no separate subwoofer, but instead they have bass elements on top of the speakers. I think they sounded really good for their price (around 120e at the time I bought it), and it beat all the Creative's shitty 2.1 systems I had used before it by miles.
PS. I'm talking about listening to music, for movies one might prefer a separate sub to get more powerful
explosions and effects and shit.