If you want to buy glasses online, you’ve got to have a prescription. Where can you get this? Check the local papers. For folks in the east, I found that Cohen’s has a coupon for a $20 eye exam. Don’t buy their overpriced glasses, of course.
When I used to live near downtown Cincinnati, I found that working class neighborhoods where most folks didn’t have insurance often offered eye exams for $15-$30.
The most expensive place to have your peepers checked out would be a suburban eye clinic. These places expect insurance and charge out the nose. I went to such a place and was charged $110 for the eye exam needed for my Peace Corps application.