My dad's 2.7L 2015 F150 was $46k. XLT trim, 4WD, Supercab (not a crew cab) short (but not teeny) bed. Has a couple of options, but minor ones, not much over what came standard with the XLT package.
The same truck today for 2018 would be about $48.5k. So your configured diesel price is not too different, assuming it is probably more tarted up than my dad's truck is (and you said Lariat, so I already know it is).
Cab configuration makes a difference too, where applicable. I suspect the Silverado will be similarly priced. The Ram you could actually configure a "real" working man's truck with the diesel engine, low Tradesman trim, with a regular cab, and a real 8 foot bed, but these would be very rare and special order for sure. And those could be had for low $30k range. Ford's decision to only offer the diesel with the two highest trim levels automatically pushes the price higher. Hard for me to justify spending $50k on a truck either, diesle or not, when you could literally buy TWO F150s brand new for that.
All depends on what you want though. The fancier ones tank so fast in value it would make sense to buy a slightly used one, let someone else take the giant money fire initial depreciation hit. But the diesel powered ones will almost certainly hold their value much, much better.