How much will it cost me to fix a roof on a home that has missing shingles and has leaked water into the house causing damage with about 8 foot holes in 2 rooms in the ceiling?

A leak this big isn't worth a repair in my book, time for a reshingle or tear off. Your can buy a few squares of shingles for under $100 and tha labor is on you but do you know how to remove old shingles and splice in new?