Another cost effective option may be contact lenses. You can usually get your optometrist to give you a "trial pair" of "disposable" contacts. These are designed for daily wear for 2 to 4 weeks and then throw them away, but if she just wears them for the time of diving and keeps them stored in the right solution in the case between dive days, the one pair will last the season. That should be long enough to see if she is really interested enough in diving to invest in a prescription mask or perhaps switch to contacts instead of glasses for daily life (14 is the age when they don’t like being seen in glasses anyway).
That said, I used the pre-fab prescription masks mentioned above for the first 10 years of diving and they work just fine (except when I set my tanks down on my first mask one time and broke the plastic where the strap attaches). Last year I switched to a custom ground prescription mask, for which I paid $250 just for the lenses, and it is appreciably better because it has my exact prescription including astigmatism and pupilary distance factored in, but I would not recommend this for a 14 year old as her eyes will likely continue to change for some years to come. With the pre-fab masks you can just buy some new lenses after a couple years. I found that vision underwater is "good enough" even with lenses that are 1 or 2 diopters off. I used -8 (the highest they offered from the time my eyes were -8.5 to -9.75 and the mask was still more than adequate to see what I needed to just fine.