I finally made it to Costco over the weekend to visit the optical counter and see about getting lenses put in my new frames. I have a very strong prescription, so having it made into high-index lenses (which are noticeably thinner than traditional polycarbonate lenses) sometimes costs a little more (sometimes it costs a lot more, depending on where the glasses are being made).
The Costco price for the lenses is $110, which is extremely reasonable. They also charged $18 because I was bringing them a frame from elsewhere. I understand why they do this, and I don't find it excessive. The turnaround is 7 to 10 business days, because the frames need to be sent to their lab in Washington state, where Costco is headquartered.
I asked if the company has its own lab, and the person assisting me gave me the rundown: the lab makes glasses for all the Costco optical locations in the US, and only for them. She told me it's the size of a Costco warehouse building, but two stories, and they make 5,000 pairs of glasses a day. (I meant to ask how they get the lenses into plastic frames, since they don't screw together like metal frames do, but I guess I can do that when I go back to pick them up.)
I'd also like to mention that the person who helped me was polite, friendly, and knowledgeable. In fact, I've never encountered anyone working at Costco who wasn't all those things.