Back in the early-1980s I bought a WWII Japanese admiral’s flag (a large one, to be flown from a ship) for $75 at a swap meet. Shortly thereafter I met a guy who would trade a wakizashi for it. The wakizashi had a WWII handle, tsuba and scabbard, which were obviously incorrect. I took the wakizashi to a Japanese sword store and found out that it had been made c.16th Century and the tang was signed. I really liked that sword.
But I was young, and I needed money. A friend really liked it, so I sold it to him for $150. I asked him a few times over the years if he still had it, and if he’d like to sell it back. ‘What? My prized possession?’ Nope. He’s keeping it.
I don’t know about a Koto period wakizashi, but a quick search turned up a Shinto period one in similar condition for $475.
Of course values depend on period, condition of the blade, condition of the accessories, maker, etc. and may reach several thousands of dollars. But given that I got my blade so cheaply (from a guy who, in the 1980s, would not trade a certain katana for a very nice classic Triumph), and that this was 20 years ago and there has been inflation in the meantime, I probably got a ‘fair’ price for my wakizashi. As I said the fittings were WWII. Also there were a couple of small nicks and scratches on the blade. Still in very nice condition for a non-serious collector, but not anything special for a serious collector.
But I still miss it sometimes. I should have kept it.