My G5 iSight iMac switched off and will not switch back on. Took into local Apple store for diagnostic, as no diagnostic LED's were lit I was told power supply was faulty. Cost £51.70. Now 11 days later I am told nothing to do with power supply but Logic board is faulty. cost £571 Methinks simply not economic to repair as I can get Mac Mini for £649 with a better spec or get cheap pc.
I would go back to the Apple Store and tell them that they misdiagnosed your problem, and since their repairs are warranted for 90 days, would they please repair your machine so that it works. If that doesn't work, then perhaps a new mini is your best bet.