The question for me becomes, "How long will it take to recovery an investment in diesel anything?"
Though it varies from week to week, diesel these days is typically as much, if not more, than regular unleaded. Right now they are about the same in my area - last night I paid $2.99 for diesel; reg. unleaded at the same station was $3.09, and premium was $3.39.
On a similar vehicle, a diesel will usually give you better mileage and much more towing power. My Ford F-250 gets 17-18mpg on average, the gas V8 gets 12-13, and the V10 is lucky to get 10-11mpg. (My dad has the same truck as me, but he has the V10, so we've done some comparisons.) I can put 3500# of camper on my truck, and another 8000# of trailer behind it, and still get 12-13mpg; dad's V10 drops to about 7 just towing his 5000# trailer. I've put 137K miles on my truck in nine years; probably about 1/4 of that towing/hauling, so my overall average is probably about 16mpg. The V8 gasser would probably be around 11, and the V10 would probably be 9mpg. Based on these numbers, dad's truck would have burned over 15,200 gallons of gas, the V8 would burn about 12,500 (and be gutless when hauling anything), while I've only used about 8,600 gallons of diesel. Even if gas averaged $2.50/gallon over that time, and diesel averaged $3.00/gal, I've saved $12,300 over dad in gas costs, and $5,600 over the V8 over the last nine years.
Diesel owners also benefit from higher resale value, when selling or trading in the vehicle. I bought my truck brand new in 2001, and the diesel was a $4,700 option. As of today, NADA adds $3,700 to the value at trade in/sale for the diesel engine, and KBB adds $4,100. Not bad when the rest of the truck has lost over half it's value!
