$9 for a secondary or old computer is not bad. No need to spend alot to protect a system that is cheap to replace.
antec 350 watt for $29.95 from antec online store
linky
I know its not 500 watts and a bit more expensive, but I have 4 antecs and never had one fail after 3 years of use.
My experiences with no name ones are pretty bad. Mostly they die within 1 year.
If you need 500 watts, you probably have alot of very expensive parts in your system.