If it helps you at all, last week i got into an '18 Ram 1500 Big Horn Crew Cab with the 5.7L MSRP of like $48k for 349 before tax. Figure out what the difference in cost is between eco diesel and the 5.7 package, divide that by the amount of months in the lease, and use that as a goal price or try to beat it. I’m sure there may be some extra room on it. The dealers near me were advertising $299 no money down on '18 Big Horn Crew Cabs with the V6 and i just shopped the deal to other dealers that had better inventory. The '19s are starting to roll in so Im seeing a lot more dealers advertise aggressive leases on the '18 stock.
Last bit of advice with where you are in the deal process; counter the dealer with the monthly payment you want to be at. Doesn’t matter at this point how they get to the number as long as the duration of the lease doesn’t change. Good luck!