Agreed with Math Geek, keyword being QUALITY.
That 550w unit you find for $30 cant actually output 350w, and when it dies there is a large chance it will fry the rest of your system with it.
Sesonic S12 and M12 are the best bang for buck right now, there is 520 and 620w versions.
Corsair CX (with silver letters, not green letters) is good too.
If you want to step it up than EVGA B2/G2/G3/GQ models are all good (not N1/W1/B1/G1 though).