|If it's the PSU or motherboard then about the only way to get at your own work is to put the HD into another computer. There is probably a suitable USB enclosure for your disk which can then plug into another computer.|
I won't try to second guess repair costs - in any event I'm in the UK so I doubt that's much use to you. If you are prepared to do it yourself a PSU is typically about $80 (varies with size and make). We can't be absolutely certain it is the PSU, but it is most likely.
Always pop back and let us know the outcome - thanks