My wife had glasses made there - took a few hours - prices were about the same as at Costco stateside. I think they all use the same grinding lab. I think the place we used was Algodones Optical. It's right across the street from the US customs re-entry line. Pay cash - some take checks, but cash always works.
Interesting, but when we first went in they made a call and said they didn't have any lens blanks of the size my wife needed. So we went to another storefront down the street. They made a call and did have the lens blanks. Then we noticed the connecting door to the store we'd just left.
We hate to negotiate, but pricing in Algodones does appear to be flexible on almost everything. We always start by saying we're "just checking prices". Whether it's drugs or booze, that line always seems to get some lower prices than posted. Might work on glasses too.
al