I'm not sure of your question. When making an offer on the house you can include a request to repair/replace the roof and make the offer contingent on the seller either making the repairs or negotiate to reduce the price to compensate for the repairs that you will have to make once you own the home. Normally, these repairs are items that are discovered during the home inspection and are dealt with through the buyers home inspection response. I usually advise buyers to have the repairs made by the seller. That will cover the buyer from any unforeseen repairs that might be discovered as the roof repairs are made. Either way the cost should be determined by getting 2-3 free estimates by a reputable roofing company.