The math would go like this, perhaps with the European fuel efficiency numbers being more believable than the EPA numbers:
Current price of diesel: $3.84/g = $1.01/liter (at least around here)
Current price of regular: $3.42/g = $.90/liter
1 km = 0.621371 miles
1 gallon = 3.78541 liters
Approximate cost of going 100,000 miles/160,934 Km in a Mazda 6:
Diesel: at 4.1 l/100 km: 6,598.31 liters: $6,664.29
Gasoline: at 5.8 l/100 km: 9,334.17 liters: $8,400.75
Given that the US version of the Mazda 6 diesel may be slightly less efficient, it seems that there isn't much difference cost wise for fuel, as long as the engines perform equivalently.
TM