It wasn't cheap at $683, but I searched for ~8 months and looked at over a hundred before I bought this '37 D 3 leg Buffalo.
When I retired from the university where I taught, I wound up with some extra cash so I decided to buy some coins with it. Over the years, I had sold coins to a local guy, and some of them were coins I was interested in owning myself. One in particular was a 1911-D quarter eagle, graded XF45 by PCGS, the big key to the series. I asked him if he was interested in selling it and offered him $950 for it, which was the wholesale price at the time (this was about 14 years ago). His response: "That sounds like too much. How about $900?" I consider this one of my best ever deals on a coin.
I had a mail-order business at the time and occasionally sold to this guy who wanted to buy some coins as investments. For his first purchase, which he showed me, he had bought something like a thousand silver dollars for $10K. Most of them were dogs. And yes, I had previously owned it but considered it part of my business stock, not my collection. By the time I retired from the university, I had been out of the mail-order business for decades. If that makes any sense.
I don't have any huge money-makers, but I do have a couple of purchases recently that I am quite pleased with. I think both were underrated, as if weak strikes were mistaken for wear. But in both cases these types of strikes are well documented. This was in an ANACS holder as MS61. I went to the registry site and found a virtually identical coin as MS65. Reference coin: Coppermania Registry Example MS 65 A few weeks ago this came up on eBay as a Buy-It-Now. I snagged it on the spot. The seller rated it as VF-XF, but I think it is considerably better. It was only $12.99.
This was like 1992 or 93... I bought a 1911 sandblasted Proof 64 NGC 2.5 IND for $21K from a dealer friend of mine. I immediately asked him to crack it out of the NGC slab, then about a month later I took the coin to Long Beach show and did the $100 PCGS walk through service and the coin came back PR65. Another time I bought a nice group of 27 gold pieces (raw $5,$10 & $20 Libs). sold two of the $10 Libs raw for $5600 ea. and sold two $20 Libs raw for $5800 ea. Yup, Those were "the good ole days"..
I got a 1942/1 Mercury dime for about 60 cents out of a pawn shop junk silver pile. I didn't realize that it was an over date until I got it home. It later graded AU-58 by PCGS. I also snagged an impaired proof 1846 braided hair cent off of eBay for about fifty bucks, I don't remember exactly. It graded PR details (environmental damage) for some light obverse corrosion. I'd call it PR-50 details. Ive since cracked it back out where it belongs.
I've told the story of the many upgrades of this coin before. I bought it as an MS-65 FBL on Teletrade for $1250. It is now an MS-65+ * FBL, and would sell for much more if I ever wanted to part with it: