You're right, Bob, if you buy the OE Brembo rotors from a Ducati dealer they are eye-wateringly expensive. But, if you go direct to a Brembo dealer, you can get better quality Serie Oro discs for less than the OE ones.
In your case, the OE Ducati price would be about £175 per disc but from, say, Carpimoto (
http://www.ukmonster.co.uk/monster/s...ad.php?t=57839) it would be around £140
https://www.carpimoto.com/EN/41281_7...r-78B40850.htm
Last year I replaced my Evo discs (OE price £480) with a pair of Serie Oro rotors for £285 delivered from Simon at WSC Performance, who matched the Carpimoto price.
http://www.ukmonster.co.uk/monster/s...ad.php?t=56141
Drop Simon an email to see what his price for a pair of 78B40850 rotors would be.