If you're looking for an inexpensive power supply without a ton of wattage, it doesn't get any cheaper than EVGA's 450W, which is on sale at Newegg for $12 after a $10 mail-in-rebate.
Shipping is an extra $4, unless you're a Newegg Premier member, then it's free. Even with shipping, $16 for a PSU is pretty cheap.
The model (100-BT-0450-K1) that's up for grabs here is 80 Plus Bronze certified. It has a single +12V rail (35A). Whether or not it's enough power for your system depends in large part on your graphics card. One thing worth noting, however, is that the 450W wattage figure is based on continuous power rather than unsustainable peak power output.
If you're not sure what size PSU you need, there are several online PSU calculators that will give you a rough estimate. EVGA has one, and there is always OuterVision, which some online PSUs are based on.
You can find this PSU here.
If you need something beefier, Corsair's CS-M series 750W PSU is currently on sale for $60, after a $20 mail-in-rebate. That one is 80 Plus Gold certified and sports modular cables. You can find it here.
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