I have two stories like this:

One thrift store had a DEC TU-58 tape drive. I bought for $20 and immediately sold on eBay for $400 (I would have kept it, but have no DEC stuff to use with it at all).

Another thrift store had a late 60s / early 70s electronic calculator that used acoustic delay line memory and a CRT for display. I think I paid $100, played with it for a while, then sold it for $600. Unfortunately, it was damaged in shipping (it was well packed), and UPS had to pay up.

I bought a Curta recently, but is was not a good deal. Prices just keep going up on them.. I wish I had bought one in the early 2000s when they were still below $500.

Other mechanical calculators are reasonable. Aside from the Curta, only the really early ones are expensive.