I picked up a vintage 40"s era Slingerland drum kit for $15. It had been painted over so the collector value was ruined. I had planned to re-finish it, but never could find the time. I sold it for $150 to someone who would give it a good home. I also picked up about $300 worth of mic/guitar cables for $10. A Kustom amp for $15. Some old mic's, etc)
I had a friend find a Gretch guitar for $25 (actually his 10 year old kid saw it and went home and told dad) it was something like a Country Gentelman that was worth about $1,000.
Mostly I've found some great old vinyl recordings (in particular old Miles, Cole, etc.)