Did a quick calculation.
12,000 miles per year
15mpg in a 5.7L vs 30mpg in the ecoDiesel
$4.25 for gas, $4.75 for diesel
Diesel would save you $1,500 a year, if prices held steady.
The engine's MSRP is $4,995.
3.3 years to recover the additional cost.
Of course if you already had the diesel, you paid less when negotiation was easier. However, fuel costs were cheaper too. And of course fuel prices vary by where you're buying.
I think it's pretty clear though, you can recover the cost of that engine option in 3-5 years easily. And you can be smug about having a smaller carbon footprint, at least until 2024 when the electric Ram debuts.