Is what's cracked your "Bell Housing" or "Timing Gear Cover". I can understand it being exepensive if it's the bell as the trans has to come out but if it's the timing cover I don't get the expense. I really don't get "if they can find one" sounds like you are being baited. Run as fast as you can. Oh, more info on your truck would help others be more helpful than I am able to be. What year is it that "if they can find one of the whatever thingies" it'll be $3200 for? I'd get the truck hauled somewhere - like a Cummins shop, that for sure would have the elusive part the dealer can't seem to locate and the expertise to install it correctly. Furthermore, if it's the bell housing how the heck do you break one of them? Is it a stick and you blew the clutch or flywheel and that's what caused the bell to brake? Or did the KDP get ya?? I'm not trying to be a smart arse (I can really be a good one - I've perfected it over the years but, to some it comes naturally not me, I had/have to try hard) Anyway hope you can get some more info that'll help make sense out of what the _teeler is feeding you.
Hang in there - there's probably someone in your neck of the woods that'll reply soon and be able to help you. Don't give the dealer another dime yet - tell them to hold off for now (make sure they understand they are not authorized to perform anymore work with out pre-approval from your lips to their ears) seek a second opinion before having that surgery performed. . . . If the dealer tells you they'll have to charge you storage ask how much it is and when the charges will start, thank them, and get a roll back over there toot sweet.
Good luck brother.