Attic Ventilation Issues

So we are in the process of purchasing a new house and it has mold in the attic. That is not a big deal because we can pay to treat that but that won't fix the problem that is causing the mold.

The home inspector and the mold guy both said there are no soffit vents but there is a gable and ridge vent. These are not having the air circulate enough and thus mold is forming. Has anyone had work done to fix their attic ventilation and how much did it cost?