sounds like power button is broken, only you know if your comfortable taking it apart. But there are videos that are good. If you think you can what do you have to lose? It ain't working now so what's the difference. I would look for some teardown videos and power button replacement vids and see if you want to try. Find out how much a button would be and then decide.
Cruzin' tha Galaxy SIII via Obsessed Rom / "INAPPROPRIATE" Kernel
Verizon Samsung Galaxy sIII - ROM Synergy nightly (awesome)
Thunderbolt - ROM SynergyRom (WOW) Thanks Virus & Everyone
Incredible 2 - ROM ? Wife (changes mind daily)
Viewsonic G Tablet - ROM Vegan Ginger
retired HTC 6800 Alltel (Verizon)
Thanks Dev's & All Who help make our devices (toys) so cool.