When it's almost time to change my prescription eyeglasses, I start looking for specials so I can save money. Last time I got glasses, the special was 2 pair for $99.00 If the prescription changes slightly, just have the old pair turned into sunglasses. They will tint your old glasses for a small fee.
Another way to save if you can't find a special is to just use your old frames. Have them put your new lenses in them. I have used all of the ideas above and save lots of money!
Saved more than that by ordering from Zenni online. Got 1 pair rimless titanium and 1 pair metal sunglasses delivered for less than $50. They came with nice cases and cloths and I will order from them again.