It starts with your prescription. There is a basic prescription and there are additions to it. This, in combination with things like your life style, daily activities, and preferred frame type, will tell your optician or optometrist what kind of lens would fit you. This is a service that you cannot count on if you select any of the cheaper alternatives.
Basically, the selection of lens divides itself into three areas:
Do you need progressive or single-vision lenses? Should they be for indoor or for sports? It is all about design.
Anti-reflective, photochromatic, or tinted? It is all about how you are going to use them, that is, your lifestyle.
Ultra-thin, high-index, shock proof, or basic? This is a mix of comfort, style and strength. Your optician will have a recommendation for you.
Certain lens materials have better optical characteristics than others. The materials can be different in refractive index, specific gravity, Abbe number (constringence), etc. These variable properties can make lenses differ in optical aberration, weight, or even make a lens thicker or thinner than another. The type of lens required for your prescription should be recommended by your ophthalmologist, optometrist, or optician. They can fit your glasses with the correct prescription and lens material. The quality of the lens especially the coating options can vary depending on where the practice get the lens from.
So, it is not only about the price. Simply comparing cost does not give you all the information needed to make an informed decision. Make sure you have all the facts available to you before you make the decision. When in doubt, keep on asking questions. The serious dealers will be happy to help you. The others won’t.