I don't know how much it is to replace an iPhone circuit board, but if your iPhone has undergone water damage, I would suggest checking out this info page (http://www.iresq.com/water-damage). After you've read up on it and determined whether or not your iPhone is fixable, I'd suggest sending it to the guys at iResQ.com. They'll do a full diagnostic test and review of your device and then fix it accordingly. I've personally never used this company before, but many of my friends have and they've had great support and success, as have many others.
Hope I could be of help and wish you the best,