Mold found during inspection period, should I proceed?

[Jason Thomas]

I am looking at a quad(2 on top 2 on bottom) which is in a very good location and put an offer on the property. Upon the inspection there seems to be an active leak in the upstairs bathroom for quite sometime now which has been generating mold and it has been found in all 4 units in the bathroom walls. The rest of the property is find but I need to find out how much credit to ask for given the active leak and mold found. I realize the drywall and some ceilings will need to be completely replaced and remediated. Anyone have a rough idea of what that could cause or the best way to approach this issue? Thanks