The cost of a prescription mask (if you decide to go that route) will depend on the strength of the prescription. My wife is pretty much blind without glasses. On her most recent exam, the optometrist commented that he has very few patients with vision worse than hers. We picked out a mask that would accomodate prescription lenses, and took it to the local vision center that handles doing dive masks.
For her prescription, it cost around double the normal cost of a prescription mask.
One thing I advise - if she has very poor vision without corrective lenses, and if you go with a mask, put together a tether system to connect her mask to her BC. We use a coiled plastic cord designed for keys for my wife's mask. You never know when you're on a group dive, when she might turn, or someone might drop in front of her, and their fin can knock the mask off her face. She will be much more comfortable in the water, if she knows her mask is going to stay within arm's reach, rather than sinking to the bottom. This is also a good idea in case the mask gets knocked off while getting back on the boat or something. Losing a mask is bad enough. Losing one that costs double or triple the regular price of a mask, because of the prescription, is worse (plus, if you're on a trip, it could mean the end of diving for her until you can get back someplace to buy a new mask with the prescription).