Best lens I've bought cheap was for large format - it was actually a twofer deal on an online auction. A Cooke Series II 10.4" f4.5 Anastigmat and a Voigtlander 11.5" f3.5 petzval as a set for $225. Both were dirty and dusty, and the Cooke looked like someone's dog had tried very hard to turn it into rawhide. But once cleaned up, the Cooke was in much better condition than it first appeared, at least performance-wise. And the Voigtlander? a ca. 1863 lens, complete with original flange and one waterhouse stop. Glass has rub marks but no damage. It's probably a $2000 lens. For smaller formats, I've not gotten any bargains. One I do kinda regret letting go was the Sigma 18-35 I had for my Contax cameras. I bought it new with my employee discount at the camera store. I've looked for another copy over the years, especially once I got back into Contax SLRs, and have never seen another copy on the used market. It was not a perfect lens, but to have all those focal lengths in one lens, especially considering what it would have cost to duplicate that with Contax glass? It was a bargain!