I believe KEH.com has a Kodak ProBack Plus with RZ ProII adapter plate for about $4k...unless someone already snatched it up.
I recommend doing a search at KEH as soon as possible.
Update: I searched at KEH for "pro back plus" and it's still there in Like New minus condition, for $4,459.00.
"Mamiya RZ67
KODAK PRO BACK PLUS 16 M/P WITH CABLES, AC ADAPTER, INTERFACE PLATE, CD, INSTRUCTION BOOK, BACK"