The antique shows here in the states, mention how much to insure a product to replace it, versus how much the item might fetch at auction.
The cost of a new knife has risen, since your purchase was made. If you were to try to sell your knife at what you paid, I doubt you would find any buyers. Most used items, especially those that are still being made, have to be discounted enough, so a buyer will feel its worth their while to take a chance on a used knife, instead of a new one.
Unless its a knife that was purchased, before the maker raised their prices significantly, i.e. Kramer, and to a certain extent Butch, it would be hard to imagine making a profit on a kitchen knife.
The most valuable knife Andy777, had in his collection was a Butch cleaver. It was not the most expensive knife. He got Butch to make it, early in his career. When Andy finally had to settle down, he sold off most of his collection. I was offered the opportunity, to buy the Butch cleaver at a fairly low price. Andy was even kind enough to send me the knife to try it out. It was a great cleaver, but I already had a enough cleavers, that I wasn't using, so I declined.
Andy had been away for the forums for a time, I let him know that Butch's work had gone up significantly, since he purchased the cleaver. I thought he should at least start the offering price at $600, which was a premium price for a cleaver. Instead he put up the cleaver for $500, and it sold in minutes. Until a knife is put up for sale, who knows how much it will get?
I don't know if the Suisin Densyo series is the same as the Densho series, but the Densho appears to be as good as it gets, before Honyaki. I'd think that 20 great years of use, out of a high end yanigaba, would be worth more then what you paid for it.
Jay