How much does a new roof cost?

210 per square. A typical 1600 sq ft house will have 22-24 squares without garage or carport. That's tear off and put back using all new flashings and drip edge, felt and a 30 year shingle.

Don't save $800 and try to nail over an existing roof. Shingles are not waterproofing and if the roof is leaking or suspect, nailing over the existing shingles is a waste of money.

If you're in a hurricane zone, code may require the roofer to re-nail the roof decking so as to strengthen the connection of sheathing to rafter. Even if they don't require it, it's probably a good idea if the house is older and especially if it's decked with plywood without hurricane clips. It could save you a good bit on insurance, as well. Require your roofer to use new lead pipe jacks and not PVC with rubber if you intend to own the house for more than 5 years even if you have to go to the supply house and purchase them yourself.