Find List of Roof Prices Here - http://tiny.cc/roofcost
If that doesn't help. Find roofers that specialize and grab some bids. Best way to find a deal.
Sometimes you can just add an additional layer of shingles right on top of the old layer (standard practice) cheapest option
Sometimes you have to remove all the shingles and put down new tar paper and shingles without replacing the wood sheeting underneath. more expensive
Then sometimes you will need to replace the wood underneath as well as everything else. Most expensive
I have a 1600 sq ft ranch built in 1956. I had two layers of shingles on my roof, had them removed and all the planking underneath was fine. I had ridge vents cut, and my total cost was about $5,000.
Since it's not really a good time to be roofing you should be able to get a few quotes (I suggest a minimum of 3) in a short period of time. Good luck.
Did you find the answer you were looking for? If not, I would suggest you get some quotes from http://www.servicemagic.com. They are prescreened contractors. I have had great succes with them.
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If you also have to replace rotten eaves and repair gutters add a couple of thousand to the above estimate. If you are talking about fancy roof or steep pitch the price increase a little.
My brother just did his roof on a 2600 sq ft colonial and he paid around 20K...He didnt go with the cheapest quote, but rather a roofer that is well established that he felt would actually be around in 30 years to honor the gurantee made. Roofers are also busier in spring and early summer so if you can wait until the fall you can get a better price.