So a couple weeks ago a friend of mine gave me a Canon EF 28-105mm F/3.5-4.5 USM lens he found in a car he bought. The front plastic that holds the lens cap on was busted and the filter ring was missing but the lens worked perfectly. I scoured eBay, found another one with a "bad" front glass element and bought it for $50.00. I used the two parts I needed to fix mine and decided to tear down the spare and part it out on eBay since it's discontinued and you cannot spare parts from Canon but it's still a popular lens.
I only have the lens half torn down and with the parts I already listed on eBay, I'll make $175.00. The first two images are the lens I got for free broken, the last two are the same lens after I rebuilt it.