Cost for 300 miles at 30 mpg gas, and diesel at 40 mpg. I have heard of light footed TDI engines getting better than 40, but we'll go with these figures for demonstration.
US current gasoline and Diesel prices
Current prices by State/City
Ok, so let's go with the national average as of 7/09/12
Regular gasoline: 3.41
Diesel Fuel: 3.68
300 mi on 10 gal gasoline = 34.10
300 mi on 7.5 gal diesel = 27.60
Let's look at current prices vs 20k miles- average mileage per year
20k@ 30 mpg= 666.67 gal = $2273.33
20k@ 40 mpg= 500 gal = $1840
Per year at only 40 mpg compared to 30 mpg regular gasoline= $400 difference, so yeah, at that rate you'd have to own the vehicle for 5 years before you'd recoup your price difference. That's probably correct due to the volatility of diesel prices vs gasoline, and the lower the price difference becomes, the lower your savings become.
I don't know many gasoline cars that actually get better than a 30 mpg average per year, and I'd be curious if diesels could do more than 40 mpg per year. I do hear some crazy stuff from TDI owners- like 50+ mpg, and in car magazine tests where the pedal is either floored or the car is not on, TDIs got more than 30 when gasoline engines were rarely over 25.
That being said. I think Mazda is on the right track for fuel cost with their SkyActiv engines. I've heard near diesel mileage from those hi compression 5 strokers- and that's without GDI. I wonder if GDI would do better, but I'm sure Mazda is either saving that, or it can't be done that way.