If you're a real enthusiast for good clean bass then I would follow the recommendations of most people here and save up for something better. If your just interested in filling in the low end of your speakers and aren't after the science, wonder, and awe of nice, clean, low bass then get one of your budget subs, it should do just fine.
This is totally true. In my world view there are basically three tiers of subs.
At the bottom you have real crap that does not really even go low, just creates more noise in the 60-100Hz range. Really only useful for explosions when all your other speakers are kinda small. These are the ones you find for under a hundred bucks. You could also say the "bass modules" people have with some HTIB sets fall into this category.
Then you have the decent budget subs, priced in the 100-250 range where now they will go a bit lower make nice LFE for movies but may have a bit of port noise (not all, the good ones should not have this problem) and are not a real tight, refined bass. For most consumers they really don't need anything more than this. You should be able to fill a decent sized room with bass.
Then above this range you have the subs that will go really low (low 20s and below) and have tons of power (you don't turn the gain past half way because it is too insanely loud). You also get tight bass and no port noise.
So keep this in mind when people start talking about how budget subs suck (usually talking about the second category). It is very possible that a sub in the second category will do exactly what you need. Sure you can get a better sub that will go louder, lower, and tighter but you have to look at your whole setup. For me my Dayton Audio 12" sub is fine for my current setup, sure I want a better sub but I would rather upgrade some other components first.
