It does not cost more. They already figured it in the quote.
3 is not done anywhere. Layering is avoiding checking the roofing plywood for weak spots and repairing as needed...with new wood. They are not doing the whole roof but repairing the base so the shingles stick.
the old roofing is binned and hauled of to an asphalt plant where it is melted down into road tar...so recycled. It rarely goes to the dump anymore.
Once the roof is proven to be a solid surface then they put on the various layers as roofing techniques have changed and no longer YOU TUBE stuff. That is the old way with the old materials. Advances in roofing tech has given better materials that last longer. Instead of 12 years it is now 30 years.
Hedging on the cost because the "angle of the dangle matters" An "A frame" roof has a steep angle and very little wall so its roofing surface is going to be greater than a roof that is almost flat.(over a 2000 square foot "on one floor" home. If they quoted 2000 square foot home, take a tape measure and measure the outside of the home LxW . If the home measures roughly 32x32=1024 square feet and you got 2 levels then it is a 1000 square foot home ACCORDING TO THE ROOFERS. But the angle matters at the peak. The sharper the point at the peak, the more roof there is. That is why they have to look at it.
. With the newer roofing materials, there is a new procedure, so the old roof shingles are tossed.
Get at least 3 written quotes. Then you got an idea for your area. Also, make sure they are insured from injury to themselves. by asking them. If not, "Ta Ta" NEXT.!