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I am pretty sure that Stu is in direct contact with Suehiro for those stones, and whoever was selling for 400, probably was reselling stone(s) bought from him.
I have not seen that stone sold online...

they are the only vendor that sell it so, yes they have the best price

I got rid of mine, I personally think those dressing stones are a waste of time, a well broken in DMT does same job and has more than just that use.

I use 10k SS before Gokumyo 20k and get superb results. I do not need pastes after, but I believe this is a matter of experience with honing and with the hones.