You can get a pair of prescription eyeglasses including lenses for $6.95 at CoastalContacts.com. They have at least a few models on sale : Dockers 138 , Metalurgy 1003 Taupe , Nikon 6006 Gunmetal, and Savvy 263 Blue. Shipping costs will run you another $6.95 for priority USPS and there is a $0.48 handling/insurance fee. So you're looking at $14.38 total. You have to know your prescription when you order.
But you can also get discounts via Ebates. With the coupon code EBATESDEAL10 you can get 10% off the order. 10% off the product cost would be $0.70 off bringing the charge down to $13.68 PLUS... you can get 7% back on your CoastalContacts order via Ebates. That would be an additional $0.95 back.
After the Ebates discounts your cost would be $12.73.
So total cost you're looking at under $13 for a prescription pair of eyeglasses. Its hard to beat $13 bucks for a pair of eyeglasses. This would make a good way to get a pair of backup eyeglasses.
I found this deal on Fatwallet.