As I know that Walmart is a cheap place to purchase prescription glasses. All the frames ranges from $18-$140, all the lenses ranges from $30-$120. You can choose the frame you like and then choose the lenses according to your requirment. But why noy ordering online, I know that there are some other online optical stores offer cheaper glasses even than Walmart. Check out at www.firmoo.com.
I think Walmart is a cheaper place to get prescription glasses. However, you will find many cheap prescription glasses online. You can try Firmoo.com. They have cheap prescription glasses with high quality. Hope this helps you.