Phaedrus, I must admit my jaw is dropping in amazement as I read your post. I know you are deeply into knives and sharpening, which makes sense for a pro chef, but spending $200 on a sharpening stone - why you could get a good climbing rope for that price!
BTW, the only reason the stone was that cheap is because it's so small. The mines these stones were quarried from are pretty much all played out. Remember, some of them have been worked of over 1,000 years! And Japan isn't that large to begin with. The Hideryama Renge Suita I have would cost thousands of dollars in a "full size" chunk. Sword polishers snatch up many of the best J-nats. It's not unusual to see rare, high end stones sell for more than many cars...
Most of my J-nats, though, are a bit more common. Almost no other single stone in my collection was over $100 for small sections.