Using what I can find for info from the VW diesel and pulling MPG #'s out my arse,
Gas, 28mpg average, cost of fuel ~$3.80 right now here.
So for you to drive the 'average' 15,000 miles/yr would cost $2,030.71.
Diesel, 40mpg average, cost of fuel ~$4.00 right now here
So for you to drive 15,000 miles would cost $1,500.00.
Savings of $530.71.
Oh but wait, we need to add the cost of fuel/water filters to the mix
O'Reilly's says they cost ~$35.00/each. I'll give the benefit of the doubt and say they can be had cheaper, say $20/each.
I'm not sure what VW recommends but changing one with ever oil change is common and IIRC VW says the oil should be changed every 5,000 miles. So you'd need 3 fuel/water filters, an extra ~$60.
Which knocks the total saving down to $470.71.
Oops, can't forget the cost of oil for the VW, which IIRC calls for 5w-40. O'Reilly's lists that for $7.50/qt (can only find it in synthetic).
The Focus calls for 5w-20 IIRC, they list Motorcraft for $4.29/qt.
Granted this is just me quickly going over things that I think are close. But it does illustrate that simply getting better mileage with a diesel isn't the whole story. The cost of upkeep has always been higher for a diesel and that closes the savings gap.
Basically from what I've figured if you take the average mileage the diesel would get and divide it by what you get now, then take half that ... that's close to the actual savings you'd see. In the above it'd go from about 40% savings to about 23% savings.
The only way I've ever been able to figure diesel is the answer is if you got very low mileage to begin with or if you drive long distances a lot. For the 'average' person it just doesn't make sense.
Don't "think outside the box".
There is no box.
Do it correct, or do it twice.