I have read quotes in the British motoring press of prices of approximately 2000-3000 GBP to 'purge' a modern diesel of unleaded petrol when it is inadvertently placed in the fuel tank. This is due to the very sophisticated and high pressure common-rail system/injectors in modern diesels. I believe it's much less to 'cleanse' a petrol version of diesel as there is no very expensive high pressure fuel pump involved.
Bish '06 RRS S/C Bonatti Grey/Ebony/Cherry with RSE; PTI; Electronic Rear Diff and Sirius Sat Radio.