How much would this cost to fix.?

depends how much you need. is your roof gable, hip, mansard, flat, shed style. How many squares of shingles will you need. What is the pitch of the roof. Standard 4/12 or as steep as a 14/12. A lot of questions come into play. And when you say tear the roof down i ume you mean a complete tear off which includes tearing off the existing osb and replacing it along with the paper and shingles. Many roofers will charge more if the roof is steep.