It depends on one roof to the other. The quality of the roofing material used, the need to remove old roofing material, pitch of the roof are some of the many factors considered for determining the cost of roof replacement. There are various other factors that are brought into consideration including roofing permit, labor, height of the building etc.
If your roof is well ventilated, then it increases the chance for it to perform better in cold. One need to keep the shingles to be used as ridge caps in a relatively warm place so that they can easily bend when required.
Yes, one can apply new shingles over the old ones. It also depends on how the roof is at the present. It is generally advisable that old shingles are removed and then the new ones are added.
If the roof is not ventilated properly then the asphalt odors will enter the interiors. The warm shingles can also cause blistering or premature aging of the shingles without proper ventilation. It may also result in bucking of the roofs.