Using a Sierra 4x4 club cab with the 5.3 gas V8 pulling our 3500# popup camper we were lucky to get 13 mpg, couldn't hold 65 on any incline without downshifting and yelling about it, and that gasser really liked to let everyone know that it was doing "work." Without the trailer it was quick and had that nice V8 rumble and driven gently could eek out 20 mpg. Same trailer, crew cab 4x4 3.5 V6 EcoBoost, same roads, same speed, 17 mpg. The 10 speed and engine quietly doing what they're supposed to do, 20-21 mpg without the trailer and without much effort on my part, felt even quicker off the line when I wanted to play. Same trailer, crew cab 4x4 EcoDiesel, 70 mph on the same route, 17 mpg, 8 speed is smooth and engine is drama free, easy to forget the trailer is even back there. 25mpg without trailer, we'll call it a bit "leisurely" getting off the line but once it gets rolling there's no issue with anything it needs to do.
There's Different strokes for different folks. Many of us buy used, so the price difference between the engines gets diluted a bit. What something "costs" and what it's "worth" isn't necessarily just about money, and can be very subjective.