Two aspects to the 'cost' calculations.
	
	
		Sure you may look at it purely at the pump. Work out the price and MPG for Diesel against Petrol for TDV6 V6 and V8. then work out what it will cost over and above the diesel. 
 
When buying second hand  - there are other items which I felt made the V8 a better proposition and I agree its a different a different set of rules for 'new car' buy.
 
When buying second hand (the more mile onboard the more significant) is the diesel has extra maintenance Costs .. diesel Filters etc but also Timing Belt (V8 has a chain) you have turbos which wear, higher pressure compression (I believe) on the diesel, which may lead to head maintenance, you have the ever present High pressure Diesel Pump and diesel injection also Fairly significant if any of these items need repair and will quickly soak up any 'at the pump' savings of diesel over petrol... 
 
OK some of these items are on both but for all my Forum Searching the V8 seems to (to date) be bullet proof, whilst the others have many reported issues.
 
Just something to consider when comparing these two options
 
hope it helps