Welcome Ian, Yes you can get good prices, however I suggest you catch a bus or cab in to Mapusa, or even a "PILOT",
I go to south Goa so Margao is good for me.
Some advise having done this last year and n times over in India.
Take your time to choose your frame, make sure the frame you choose will take the lenses you are fitting- I know this is obvious but in India they will sell any thing you want and then you will have the runaround when they have a problem.
Service is normally quite quick to get the glasses done, so do this soon as you reach there so you have some time to get use to them and if there are any problems you can go back. If you are buying more than one pair- dont wait till you receive your first.
Although they are cheap they will try to make them cheap too, so if you want quality, ask about the type of lenses and also the thichness etc.
Last year I choose the frame, went to pick up, the lenses did not fit the frame properly so I said I was not happy, had to choose another frame that could take the lenses, finally compromised on another, had to go away and return to pick the new glasses again, I asked for more discount and got some. I paid approx £18 in the end. Similar pair in turist area £60.
I was happy with the final result and the people! but am going back this year for another pair- I lost them