A credit for 50 percent of the cost of a new roof seemed reasonable. Sometimes an agreement can be reached by obtaining estimates from several contractors. Contractors' prices can vary widely. A lower bid can help put a deal together. Often it's wise to get a second opinion.
Inspectors sometimes make mistakes, or err on the side of caution due to liability concerns.
If the sellers give you a credit for repairs, make sure you get the work done. Otherwise, you could be stuck with an even larger repair bill when you sell.