The cost is related to the material of the lens that you choose. If you have strong prescription, you may choose high index lens which is more expensive than regualr lens. Because high index can make your lens thinner if you have very strong prescription.
No. The cost has nothing to do with your prescription, but with the lens index you choose. It is the same like shoes in larger size are the same price with smaller size. The thinner lens you choose, the more they cost.