3Gs Power Button Not Working!

My phone was water damaged several months ago. I kept it in rice for over a week and all started working fine until today, when the power button is not responsive. Is there a least expensive way to correct this problem? Do I just simply need to purchase a power button and install it to fix the problem? I have opened an iPhone before, or you think it might be the motherboard, even though everything else works perfectly?

Any suggestions would be greatly appreciated!

Thanks!

 

How would a person know when a motherboard is damaged by water? Everything else works perfect, it's just the power button that's not functioning. I just don't want to be ripped off.