Yep! that'd be me ... :)
Over the last two years, and approx' 35,000km, averaging 15.22c/km for diesel (Td5 D2a auto) - on/off road
Approx $2600 PA for fuel. ($50/wk average)
over 100,000km works out to be $15,220 ($8480 cheaper) than your V8 ... 100,000km would be approx' 5.5 yrs for me.
When we were looking I did the sums, and with the intention of keeping the D2a for 10-15yrs, and paying 4-6k more for the diesel, the payback period was approx' 5yrs ... then the next 10yrs are cheaper. Servicing costs are going to be pretty similar.
A gas V8 is cheaper to run than the Td5 (and a V8 + fitting sequential injection gas would have been around the same cost as we paid for the Td5) but the lack of range makes it unsuitable for touring ... if it's a weekend play toy then a V8 on gas would make sense.

