We purchased a Persian kitten at our local pet store. When we took her to the vet were were told she has cataracts in both eyes we paid $400.00 for this kitten. What are the chances of having her going blind?

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Your Kitten is likely blind right now. The kitten you were sold was most likely born with cataracts. It was wrong for them to sell a kitten with such a defect and it is likely not the only one affected in the litter. The lens of the eye is normally transparent and devoid of blood vessels. A Cataract is opacity of the lens, either due to hereditary factors, incidental malformation or because of other factors affecting the lens. Because the lens is opaque, it cannot be seen through, therefore resulting in, blindness.
Genetic Cataracts: Neonatal (around the time at birth) cataracts which are probably genetic are seen in Persian cats. Although a genetic etiology is the most likely cause of cataracts in a pediatric patient, other considerations include trauma, maternal influences such as infectious, metabolic or toxic disease.
Another possibility would be nutritional cataracts: Associated with feeding Esbilac® or KMR® milk replacers, evaporated milk formulas, or goats milk to newborn puppies or kittens. Presumably due to relative deficiencies in either arginine or phenylalanine. Only 50-75% in a litter may be affected.  Lens changes are generally mild to moderate and do not appear to occur in animals which were able to nurse for the first 2 weeks after birth. Once affected animals are weaned or placed on nutritionally complete diets the cataracts often disappear, but an opacity may persist.
The cataracts can be removed, which will likely restore adequate vision, however I this can be extremely costly. You should consult with a veterinary ophthalmologist in your area for a complete discussion and evaluation. Your vet should be able to recommend one in your area.