Richard Kettlewell

Isn't this the sunk cost fallacy - essentially, the repair cost is a deal on a second-hand lens, and the fact that the deal involves returning your uselessish lens-shaped object is a minor detail. So the limit on the repair cost should be the most you'd be prepared to pay for a lens of this sort from eBay. And if it turns out you don't actually want the lens, if the repair costs less than that it's still worthwhile because that's the sum you'd get back selling the fixed lens on eBay.