I work at a dive shop part time and had the same problem. I had an astigmatism and had to get custom lenses from a great dive mask company that a lot of scuba shops represent called Seavision www.seavisionusa.com.
My mask ran about $170.
A lot of scuba shops sell masks that have interchangeable lense in preground diopters if you don't have the stigmatism thing going on.
Some fairly common brands of masks that have this option is Tusa or Sherwood. www.tusa.com
or sherwoodscuba.com. The lenses are about $60-$80 a set plus the cost of the mask $50-90 on average. If you are buying more gear such as fins, snorkel, shops will often give you a discount off the whole deal.
I would highly recommend finding something at home that way you can have the correct rx and a mask that has been properly fitted. Nothing worse than a leaky mask. Also, pick up a slap strap for your mask. Makes all the difference. Post back if you need more details.
If cost is a factor and you won't be using the mask much. I know that Blue Hole Dive Center on Front street in San Pedro rents masks with prescription lenses for $5 bucks a day. Laurie and Ken the owners will take good care of you. Their website is www.bluedive.com
Hope this helps.