iMac dv400 booting up issues

i've had my iMac for about 4 years now, and just recently it's not wanting to boot up at all.

i had the battery replaced, and still nothing. it'll turn on for a split second then shut down rather quickly.

would anyone have a general idea on what the issue may be? and if so, would there be a cheaper method than the 300+ dollar estimate the guy told me? (part estimated to be about $200 + $100 dignostic fee)

it's the powersupply, if you have ever taken apart a computer you can probably replace it yourself. Here is an example of the part you need (this one is for the 333mhz imac) http://www.mac-pro.com/s.nl/sc.2/category.114/it.A/id.770/.f