About 1965 I was sent by a friend to a store called (by the kids in Dundas, Ontario) the WE BUY SELL ANYTHING JOINT. There I found a 1918 Martin bowl back mandolin for $20. I kept it till about 30 years ago, when I traded it on a 1962 D-21. I think I was allowed about $250 on it. Unfortunately I didn't get a great deal on the D-21, but I still have it and love it.
About 15 years ago I was helping my son pick out his first guitar when I spotted an old open back, no-name banjo with a bird's eye maple pot and a pearl star on the peg head. I ended up paying $2o or $25 for it, and it's one of my favourite clawhammer banjos. I've been offered $500 for it and turned it down.
I've also gotten good deals on Autoharps and guitars, but I've also probably overpaid for enough instruments to make up for those.