You can't predict the damage you are doing to your eyes with either ill-fitting, poorly-made or both glasses.
I've heard people complain about the price of Costas or any other premium made glasses. You know what they have never complained about after using them? Eye strain and vision problems.
You buy insurance for your cars, boats, health, etc. IMO, this is like purchasing insurance for your eyes.
"Oh, I'm clumsy and I tend to drop things in the water / sit on them / allow the dog to eat them / have them stolen by the Loch Ness Monster." Obviously being facetious toward the end but you get the point. If you spend money on these things and still treat them with the disregard of a pair of aviator glasses you picked up at the gas station, then I dont know what to tell you.