I haven't had any real serious scores in the three-figure range, but I've had a few nice ones. Probably the best deals I can think of came from guy from the former Yugoslavia who sold records and stereo gear at my local flea market until a year or so ago. He didn't quite know what was what, so he wanted $20 or more for anything by the Stones, Beatles, Led Zeppelin, Pink Floyd, etc., regardless of condition.
Meanwhile, he sold me a M-, still-in-the-plastic, gatefold-never-even-opened, original US pressing of Caravan's In the Land of Grey & Pink for $5.00. I sold it to a buddy for $40 about a year later.
I also got a copy of the second album by an obscure Spanish prog group called Iman Califato Independiente. Again, M- and $5. I'm not quite sure what it's worth, maybe $25-$30, but I'd never heard of it before, and musically it's a gem, and that's really what matters.
Another good one is from the early days of eBay. I got a UK pressing of Centipede's Septober Energy on RCA/Neon for $6 because it had no picture and I think it was listed under Robert Fripp, who produced it.
SFH Records - prog, fusion, folk & more: http://sfhrecords.gemm.com/