Good luck finding something that cheap. Every now and then, NewEgg, Directron, Outpost, Frys, Zipzoomfly, and TigerDirect will sell some units. But I would not trust any power supply that sells for under $100...
Take a look at the Power Supply guides published on this forum. I don't agree with everything posted, but it has a very helpful list of what power supplies are good, and which ones to avoid.
As for the 500 Watt to 600 watt, that is about right, although the decision is more about amperage than watts... and steady output, and reliability.