Most Expensive: Maybe not by the numbers, but "hurt the most": a tie between a beautiful little Canonet QL19 G-III that, after only running one roll of film through it, was stolen...or re-buying the Nikon 70-300 VR that was stolen along with that Canonet. I'd have re-bought that Canonet too, but that's easier said than done.
Best Deal: Another tie, both from the local Craigslist...either a near-mint Canonet QL17 G-III a guy had in his garage collecting dust ($20), or a similarly near-mint Nikon FM2n a kid was selling because immediately after he bought it, he realized the film advance lock wasn't working (making it possible to skip frames). When we met, I asked him to show me the problem, and he did...while specifically depressing the multi-exposure lever. Apparently, he'd not even run film through, or else he'd have realized his diagnosis was incorrect. I'd have set him straight, but the whole time, he had this attitude as if I were paying him for trash that he'd ahve otherwise thrown out.
So I kept my mouth shut and bought a 9.5/10 chrome FM2n for $50.