At the height of the c-mount lens craze, before all the fast native lenses were available, I won a lot of 30ish lenses for $1,900. The lot included 3 Angenieux 25/0.95 lenses that I sold for over $1,200 each. Fun times.
I don't think of bargains in terms of resale value, but in terms of bang for buck. So my best buy was a brand new GX7 with kit 14-42 ii lens for £250 from a UK high street store that was having a sale to clear stock for the GX8. It's what got me into M43 and I've carried it everywhere since. I sold the lens for £65 making it an even better buy.
Of course if I hadn't bought it I wouldn't have also bought an EM1, panaleica primes or pro zooms, so best bargain is relative! I also don't think my photography would have developed as fast had I stayed with DSLRs, as the ease of carrying, simplicity of the technical elements, and sheer fun factor has driven my passion over the past year.
Around 1970, I got a 4x5 Speed Graphic at an auction in upstate NY for $5. It was this just this odd black box that nobody else could figure how to open. I opened it about 0.5" and realized what it was. A year later I got a matching 4x5 enlarger for $15; it had a massive condenser lens. I still have the Speed Graphic but the enlarger was left behind in a move.