My mare has cataracts in both eyes, she is 29yo so sadly the op was not an option id consider for her so cant help you with costs of surgery, but can tell you my mare copes really well i make sure she knows where the water trough is if she goes in a new field and she does not wear fly masks any more, she just wears fly fringes so not to obstruct her vision any more than necessary.
Hope that helps.
My Arab gelding has a cataract in one of his eyes, due to an injury before I bought him. I have never considered having it removed as he copes very well as he is.
My friend's TB had a nasty field injury - a kick to his face - that caused a small cataract in one eye. She has the vets to check it regularly and they advise that it is not growing so to leave well enough alone for now. They say he may have a small vision disturbance but if so it doesn't upset him at all. In the unlikely event that things deteriorate, they have discussed the surgery which she is advised is relatively new but has a good success rate. No idea of the cost but I guess it won't be cheap!