First, I wear bifocals, which are more expensive than regular lenses, and they still aren't $300, much less $900. I find that number hard to believe. Sounds like salesperson math.
Second, I adamantly advise against ordering prescription glasses from online places like Zenni. I ordered a pair once and they frames did not fit my face AT ALL. I don't mean I didn't care for the look. I mean the frames were too short and gave me a headache after only minutes. I tried to have them adjusted at the mall, and there just wasn't enough material there to adjust. Cheap glasses aren't cheap if they don't work!
Finally, I have eye care insurance, and it pays next to nothing for glasses. I get a better discount from a professional group of which I am a member. Check everything in your wallet: AAA, AARP, credit union, whatever else you have. Quite a few places offer small discounts.
BUT the insurance is still worthwhile. It's pretty much a wash -- it's cheap insurance, and the amount off the glasses is cheap. But IF you ever have to have any type of surgery, you're covered. Many of us will have cataracts as we age, and insurance will cover that. A relative of mine just had MAJOR eye surgery . . . and then had it again on the other eye. That's when your eye insurance is worthwhile.