Hi: This is heart breaking indeed. So, the bad news is that cataracts have begun to form, especially in one eye. The good news is that your horse still has sight out of one good eye and some sight out of the eye with the cataract. Research on Can-C reports that application of the Can-C eye drops as directed results in about 95% improvement in the subjects that were studied. Subjects have been dogs and humans – not horses – but researchers concluded that the Can-C helps reverse cataracts in all mammals including horses in, again, 95% of the cases. So, it is certainly worth a try, especially to insure that the good eye remains clear of a cataract.
Prostaglandin analogs may cause changes in iris color and eyelid skin, growth of eyelashes , stinging, blurred vision , eye redness, itching, and burning. Beta blockers' side effects include low blood pressure , reduced pulse rate , fatigue , shortness of breath, and in rare occasions, reduced libido and depression . Alpha agonists can cause burning or stinging, fatigue, headache , drowsiness , dry mouth and nose , and also they have a higher likelihood of allergic reaction. Carbonic anhydrase inhibitors may cause stinging, burning, and eye discomfort. 
Some of the generic drops have become expensive because of consolidation. For example, generic pred forte drops used to be very inexpensive but now there are only two companies that make it and those two companies are owned by the same two large pharmaceuticals that own the branded medicines, hence the price of the generic is almost the same as the expensive branded options. The price of drugs does vary from one pharmacy to another and each insurance plan has its preferred drugs which cost much less for patients depending on the deals they have made with the manufactures. For example brand A might be priced well at one store because they have made a good deal with the manufacturer and made it their preferred drug while brand B would be expensive. Across town a different drug store chain might have a great price for B and a high price for A because they have make B their preferred drug and have a great volume price with the manufacturer.
Alas, it is hard to find a cost effective way to protect your eyes around cataract surgery.