The reason these knives cost so much is the combination of steel and labor. ZDP-189 and Cowry X are probably the 2 most expensive cutlery steels on the market next to real tamahagane. Not only that, grinding steel with this much wear resistance literally doubles and triples the time required to grind. Most of these steels can not be quenched very thin due to the possibility of cracking and warping inherent with carbon levels this high. I mean over 2% carbon is really considered cast iron, it's not even steel anymore LOL! But, anyhow, you are paying for a lot of extra labor and much more expensive material that's often very finicky to work with. This adds to the rareness as not many smiths are willing to work with these steels. So, it's a combination of things, not just the steel. If all steel was as easy to work as the next then we'd all be using REX121 and K390. (I wish)
My personal experience with ZDP is that it is closer to HAP40 than anything else I've tried. You certainly notice a marked increase in stiffness at the edge compared to the "lesser, weaker, inferior" steels out there, because it is true, all of our childhood fantasies about ZDP are true, and it really is adamantium! LOL I had to, I had to, sorry!
It's damn good steel, gets very sharp, and holds that "off the stones" feeling for a good while longer than most. That's the biggest thing I notice with these crazy steels is they don't roll for anything, so you keep that fine edge pointing down much longer. Also, back when ZDP came out the other powdered metals were still suffering from "carbide fallout", while ZDP did not, making it seem even better at the time comparatively IMO, adding to its mystique. Basically, ZDP really is an exceptionally good steel, but gained an even bigger reputation. Even though it has been out longer than most of the newer super-steels, it's still among the highest performance steels available today.
And on that note time for bed, I feel like my posts are making less and less sense as I go lol