Jen covered this for the prescription medicine case; I wanted to call some attention to its general applicability.
One of my failures of frugality was the time I bought from my eye doctor clip-on sunglasses that were fitted exactly to my glasses. They cost a lot.
Yesterday, I bought generic clip-ons from Costco for $13. They fit my glasses well, and look fine. (They're not flip-up so don't have the flip-ups' bulky hardware over the bridge of your nose.) If you were to look very closely, you'd notice their inexact fit, and the small spring that makes them adjustable, and I'm fine with that.
Whenever you're faced with a pricey purchase, make sure you know what the alternatives are.