You need to complain to that little weasel's boss
If he doesn't have a boss, write a very nasty letter to corporate headquarters. Not only did he rub it in about the high cost of the medication, he gave you false information about it. The latter is the big deal here.
Eye medications are shockingly expensive and they do have a very limited shelf life even when there are preservatives in them. Part of the problem is that any liquid that is open to air is likely to start growing bugs after a while. This isn't critical for medications that are going to hit the hydrochloric acid in our stomachs but it's highly critical in things that are dripped into the eye. Infection and ulceration can happen very quickly.
Add to this the fact that many antibiotics out there are still on patent and that means another hit to the wallet.