Prescription glasses pries can vary depending on the prescription needed.

According to Clarksvill Eye Clinic's article on prescription Sunglasses, sunglasses are more expensive than regular perscription eyeglasses. This same article, however, says that sunglasses are only slightly more expensive, and they protect better than transitional lense glasses.

Accutane, which can be covered by insurance.

Not legally. But they prescribe it for just about everything, and its not terribly expensive.

Since you don't specify what the prescription is for, it's impossible to say. Most likely it's difficult to produce and/or covered by a patent.