Ford F-250 with V-10
Perhaps, if we compare new trucks, there is only a $5k increase in the purchase price for a diesel engine, but in used trucks, the diesels command a preimum of nearly $10k. There are good used late model dualie trucks with the v-10 that can be bought for around 8 grand and very similar trucks with the diesel go for about $18k. I'm not knocking the diesels in any way. Like I mentioned, the one I borrowed I really really liked. In fact, I hated to give it back. I loved it's power (torque actually). But sadly, I don't drive it for pleasure, I just need to move equipment, and only drive about 35000 miles per year, plus when I'm done with the truck, it's one tired vehicle, even if the engine were to be like new. But in terms of dollars per mile the best value to simply get the job done with a used truck is the v-10 simply because they have the reputation for getting crummy milage. If we look at the savings in terms of fuel costs (which are only a portion of total operational costs) you'll see that 9 mpg = 33 cents per mile and 7 mpg = 42 cents per mile.
Therefore, I think that Mr. Idaho will be pretty happy with his purchase paying 23 cents/mile (13 mpg @ $3/gal) vs 16 cents per mile (18 mpg @ $3/gal). Even if his great deal is the standard $5k, he will have to drive over 70k just to break even. He will lose out on resale value though as the gasser will depreciate like a wild thing.