I actually own the last Prototype together with a Prototype Pre! It seems disappropriate to discuss money on the web, as anything said, will never disappear; I feel free to describe my feelings about this gear, which in truth, tells more about me, than about the gear - right?
Beside that, I feel, your mentioned sum seems reasonable, as it is an exceptional amp all the way, but, as I said above, I felt that already about the ZH270:
270 is used in my Studio with a Wadia S7i and Stella Opus/Novus Speaker system, they can use the added oomph, and the slightly darker sound of those speakers, whereas the ZH230 goes REALLY well with my VSM MXE.
I can add, although Berning amps never go down, mine did: Or rather a power tube failed. I just want to be fair and give context.
It is supposedly easy on Tubes, but hey, we all die!