| RT here. This is an excellent question.
First off, I currently have 4 pair of glasses (I purchased them during my September visit):
2 bifocal rimless
1 distance rimless
1 reading with rims
Total cost, including exam, was US$110 (I was quoted/paid in Soles). The girl apologized because they wouldn't be ready until 3:00 pm (it was 10:30 in the morning). I told her that I would return the next day; they were there waiting for me first thing in the morning.
Crappy Spanish was/is not an issue. 'Minimal' is plenty. It's not like you're discussing world politics. It's their job to get you examined, shopped, and selected. And you've bought glasses before -- the questions are the same in any language.
I didn't remember the street name offhand, but Spinetta (above) says that it is Rufino Torrico, and that rings a bell. What I did was ask a cab driver if he could take meto the street in Centro with all the glasses stores (Senor, yo necissito lentes. Tu sabes la calle en El Centro con muchas tiendas? - or something like that). They all know it: it's a two- or three-block strip with nothing but glasses stores (and more hidden ones). They're all seemingly identical, and when they don't have any customers, the employees stand in the street and try to talk you into buying from them (think Pizza Alley with Glasses). What I do is wander around until a pretty girl tries to sell me glasses, and that's who gets to be my salesperson.
You don't need me to tell you how to negotiate, just get prices up front. I've always been quoted one price for the frames, and then a separate one for the lenses. Tell them that you need to get an exam, too. They don't all have eye doctors, but they 'share' the few of them that are around. The girl will wait for you during your exam to make sure you come back to place your order with her.
Be sure to ask for a copy of your prescription -- it saved me a lot of time when I needed a replacement pair in a hurry!
As I said, I've had no problems, and it is both a very un-touristy thing to do (allowing me to pick up some local color) and it puts me in a part of Lima other than Miraflores/San Isidro, so I get to take advantage of that and go whoring in a different direction than I would normally. Or I just walk around a different part of Lima and see what I can find!
Hope this helps. | 
