Screen wont respond. Lag when starting phone.

Possible water damage to my s6 edge. The screen no longer responds to touch. It also takes for ever to load after restarting. Sent in to virgin for repair they say it's water damage and isn't covered by warranty. Went to phone repair store he opened it up saw some damage on daughter board connecter. Switched out the MB and screen was working. Unfortunately the MB he has is from a demo phone so that won't work as a replacement. Is there any way to fix what I have or am I doomed to have to replace the board. If so how much am I looking at for the replacement?