It's depends. There are many kinds of eyeglasses lenses, and different lenses have different prices. Besides, the prices of the glasses lenses also different in every shop. As far as i know, Firmoo.com, an online optical shop, offers free Standard (1.5) lenses, and $9.95 for Thin and Light (1.57), $19.95 for Thinner and Lighter (1.57 Aspheric ) and $26.95 for Polycarbonate lenses (1.59) etc. Want to know more, you can have a look.
I think you can only have a look at a nearby eyeglasses shop because online glasses stores can't provide you with only a pair of lenses. If they could do that for you, you also need to ask someone to cut the lenses to fit for your frame. So you'd better take your frame with you to consult the optician in any eyeglasses shops. Maybe it will cost you less than one hundred dollars. Good luck!
That depends on your degrees and which kind of lenses you will choose. Once I bought a pair of eyeglasses in an eyeglasses shop, the optician charged me for $89 and my friend for $100 though we had the same kinds of lenses but different degrees. But later some people told me that the optician overcharged my friend. But at least it is true that different lenses are at different prices. So you'd better ask the optician how they charge for different degrees. Hope this helped!
If you can buy a pair of lenses online, it may cost you less than $50. But you may need to ask a local optician to cut the lenses to fit for your frame. Maybe this will cost you at least $20. In all, it may cost you $70. If you buy a pair of lenses in a local optical store, the optician will cut the lenses for you for free. And it may cost you less than $70 dollars while it depends on which kind of lenses you may choose.