when the buyer put down the offer, they were expecting a functional working roof. When the roof is in their end of life, the vaule of the house is greatly reduced. In this regard, your house probably does not worth the price the buyer has offered.
A new roof will not add the value of the hosue (most appraisal will ignore the new roof). But a brand new roof make the hosue much more attrctive and it also reduce your home owner insurance. In this regard, you can ask more money.
As for how much more, ask your agent. But half of the new roof cost is too much to me.