I have purchased exactly one prescription mask ground just for my prescription. That was in 1998 at a cost of just under $500.00. Since then I have gone with the "off the shelf solutions", and been very pleased, at a per mask cost of about $150.00 to $200.00. They have been "close enough" for underwater use, even though I am now a trifocal wearer. I have put in a small "reading lens in the corner of the left side, but can get by without that.
So in my experience, the "off the shelf "solution works just fine. My LDS does not carry them, but another shop about 7 miles away does. I am sure you can locate one in your area.
DivemasterDennis