The resale will definitely be more, but I doubt it will remain dollar-for-dollar over the lifetime of the truck, so I think $3,000 is aiming a bit high. Maybe $2,000.
I just checked KBB on the trade in on a 2010 RAM 2500, CC 4x4 8' box, Longhorn with 100,000 miles, equally equipped 6.7L vs. 5.7L, and here's what it showed for trade in value:
Diesel: $23,620
Gas:$18,145
That's a difference of about $5,500. The option cost to get a diesel over gas in a 3/4 ton is about $8,000, so use that as a guide I guess. So, I'd say the $2,000 I quoted above is likely what you could expect after 4 or 5 years.
Now, the mileage factor based just on fuel consumption, will largely depend on how many miles you drive a year. For the sake of this example, let's say you keep the truck 5 years and put 100k on it; that's 20k/yr.
So, 100,000 miles at an average of 28 mpg (and you won't average that, but I'll just use what is "published" for hwy mpg) would equate to about 3572 gallons of diesel, and at a hypothetical $3.97/gallon (national average as of today, which is 50 cents less than what it is in my region) equals about $14,180 in fuel expenses for those 5 years. DEF not included, but I'll go with what was already posted and say $450.
Now, gas truck (5.7L Hemi): lets say you get 20 mpg average for same number of miles at national average for mid-grade ($3.56/gal) = $17,800 in fuel for 5 years, or about $3000 more after you factor in DEF costs. Seems as if you have broken even already, right? Paid $3000 more for truck, save $3000 in fuel....ahhh, but how much did you spend on maintenance? Let's see.....
Diesel: oil changes every 10k at $60 each = $600, plus fuel filters (at least 5 of them, likely double that) at $15 each = $75 to $150, so total is let's say $700.
Gas: oil changes every 5k at $40 each = $400. No fuel filter to worry about (it's in the tank, on the pump). It, as well as the spark plugs, should last 100,000 miles (not that I'd leave the plugs in that long....but hey, if you're trading it in, why replace them?).
So, now the diesel has cost you about $700 more out-of-pocket based just on these calculations, and this is best-case mpg figures. Also, when you trade it in, you will "lose" another ~$1000 on the trade in value vs. the additional cost for the diesel option (and this assumes a cash purchase or 0% interest rate - if you are paying interest on a loan, add that expense as well into the "loss" figure).
Ok, now some additional costs that you may incur that are not calculated here: fuel additives, tire cost (the truck weighs more so will therefore wear tires more quickly), insurance cost (the diesel will likely cost more to insure), and out-of-warranty repairs (if you keep it longer than 5 yrs/100k).
If the spread between diesel and gas fuel prices is more than the national average (as it is here where I live), then that fuel savings dwindles away quickly. In my example above, it would have added about $1700 to my (local) fuel expense, negative over half of the "fuel savings"!!!
So, as I've said before, I'm not so sure a diesel is more cost effective over the long term. Unless you tow frequently.......or keep it literally forever (and then the trade-in value advantage is next to nothing).
Just my thoughts and observations.......doesn't mean I'm right.