All I can tell you is, rear disk is thicker than front one. So not only the diameter, but you need to pay attention to the thickness of the rear.
My 750 have all three at same diameter, but rear is significantly thicker (i8f I'm not mistaken, rear min. thickness is at 6mm, where front is at 4.5 mm.) so just be careful when you look for the replacement.
Also, parts are too expensive? Try living middle of no where. I can't get this, They won't ship that even when I'm willing to pay for the shipping. On top of that, Amazon is the only supplier willing to do free shipping. I have to pay for parts + shipping, usually 2nd day air price. Why? I have no f**king idea. If you go to UPS or FedEx site, they do offer ground shipping, but when you call the supplier, they all tells me they don't have it! And for some reason, they won't ship USPS.
Lastly, I'm a fan of 750's. So I could be wrong for the bigger Kawasaki's.
82' KZ750-R1 cafe racer style. Clip-on, Rear set, Fork-Brace, Mikuni Flat-slide (forgot the size), Kerker.
84' GPz750 (basket case). everything are in pieces.
89' ZX750-H1 (ZX-7 Ninja). Resurrection project are on going with my KZ750. Everything is stock.