Kharon
Contributor
When I first started diving my dive shop sent my mask and my prescription from my eye doctor in. Custom lenses were made and they were fused to the original mask - fairly expensive. As luck would have about 6 dive trips after I dropped it on the floor of the liveaboard and it shattered. I did some shopping around and TUSA made a mask that fit extremely well and they had lenses you could buy and pop right in, my dive shop carried the mask and the lenses. I went this route because if my prescription changes I can just change one or both of the lenses without replacing the mask. Big bonus - much less expensive to go this route as well.
Doesn't work if you have astigmatism. Even less so if it's different eye to eye. I have no choice but to get custom lenses put in. For the perfect vision it provides it's worth the cost - and I coddle my mask.
Considering the cost of all the other equipment (regs, BC's/backplate-wing, etc.) the cost of custom lenses is quite a small percentage. It's your interface with the underwater world. I can't believe in skimping and seeing things less than perfectly clear.