I was talking to a friend about this, and she said that she generally expects to pay about $250 for a pair of glasses. So, two pairs would be about $500. Well, thought I, I know my dad got his eyes checked and the prescription filled in Mexico City for $50. So:
Flight to Mexico City: $250
Hotel for 2-3 nights: $40-60
Eye check and two pairs of glasses: $100
Leaving me with approximately $90 spare money to do fun stuff in DF. Tempting!
(Yes, I know I can get discounts on frames and second pairs of glasses and stuff. But you're not supposed to factor that sort of thing into account when you're considering a spontaneous and spur-of-the-moment adventure!)