Pre-recession an SD40-2 that was RTR could get $200-250K. I seem to remember reading here that PAR picked up their last batch off Helm for about 70K. Helm recently scrapped a bunch of motors, mostly SD40-2's. These, presumably, could be purchased for barely more than their scrap value because as bulletproof a workhorse as an SD40-2 is, there are thousands across the continent in lease fleets or being prepped for retirement. Their just isn't the demand to warrant the 200K price tag anymore.
Figuring 100K for an SD40-2 and 2.5 million for a new AC model would make an SD40-2 1/25 the price of a new unit.
Add to that less tech components, ample parts supply and, again, the bulletproof reliability that new units often can't match and it makes any fuel saving, I would think, relatively minor.