Bottom line, don't buy a diesel for the fuel mileage, although it's a nice little perk.
Buy one because you want one, need one, or so you can get you b@lls back out of your wife's purse!
On a more serious note, yes the $8k upcharge new is significant, and not for the weak wallets, but those that claim "increased cost of ___, and loud smelly, never get your money back...etc" are typically gas HD pickup drivers who won't admit they wished they had double the torque and could actually get double digit mileage numbers doing anything but running empty down the highway!
BTW, the naysayers can show me where any diesels, save for the old turds and the 6.0 Fords and to a small extent the 6.4s, don't recover most or all of the upcharge on resale compared to an equivalent gasser. You won't be able to. You'll get your money back when you sell it.