I can give you a material cost on metal. Right now in my area we get 29g metal by ABC manufacturing for 2.62 a ft. So if you need panels 10ft long just multiple 2.62x10 and they come to $26.20 a piece, they are 3 ft wide. You can for sure go over shingles, I have done a few jobs that way. Some people add felt paper over the shingles to protect the backside of metal when the metal expands and contracts from temp. I did ask vendor and they said felt was not required and without it did not void warranty. On a 10ft pc of metal you would use 24 screws and I believe they are $.08 per screw. You can probably find better prices at the big box stores but none close to me and the service I get is outstanding so I do not bother driving 100 miles to get discounted pricing,,,usually! Keep in mind there should be a mark up on material depending on the contractor for handling and profit. I try an get 3.62 a ft but I have little overhead.
Labor I cannot help you with as I am not sure of the going rates but the material prices I listed should give you a ball park. Oh yea, you'll need ridge cap and thay are about $21 eack pc at 10.5 ft long. Some people like the look of gable trim, I don't but they are about $25 a pc, 10.5 ft long. Gable trim is a outside corner pc that sits on the roof and facia. It does protect the corner but I usually hang the metal over the corner 1" and 2" on the lower eves. if you have a ridge vent they become expensive. Menards sells a black foam adhesive back strip that you stick to the underside of the ridge cap that allows heat to escape trough circulation between the soffit and ridge. Menards price was 4 bucks more than I get them locally. Locally I can get a vent strip for $17 and menards was 22 but was 12 ft long. You need 2 vent strips pf pc of ridge cap (one on each side). Some people like closure strips all over but I usually use them in valleys. Somethimes I use a closure strip on a more shallower roof pitch like yours if we are not using a ridge vent. Shallow roof pitch would need a closure strip to keep water out on a strong driving rain when no ridge vent is being used. Closure strips are like $4 for a 3 ft pc.
