I haven't seen anyone mention Costco yet. I have been getting Costco glasses since the mid-1990s. I went to other shops a couple of times when I moved away and didn't live near a Costco, and both times I wasn't satisfied. I have two pairs of Costco glasses now (distance and reading - and yes, I'm only 32) and the costs were $110 and $90, including lenses (but without anti-glare, since I find those hard to clean.) These are single prescriptions and they'll charge more for bifocals, but they are still cheaper than most other places. The one downside is that it can take 1-2 weeks to get the glasses, so this is no good if you need them right away. If there's no rush, check them out, or maybe another of the big bulk retailers.
That said, two years ago I was unemployed so I bought a pair of glasses at Zenni. For $20 I got a pair that didn't feel sturdy, but it's lasted 2 years so far, so that's good enough for me. I switch back and forth between these and my Costco glasses - I need to have two pair so that I have a spare, just in case one breaks. I may go back to Zenni for my next pair - the price is amazing, and for $20 or so, even if they don't last, it's worth a try. The clip-on sunglasses weren't very good, but luckily my old clip-ons happened to fit, so I use those.
For those of you spending $300 per pair, consider some of the alternatives. Unless you're one of the lucky ones who don't need new glasses often, you could end up spending a lot of money very fast.