Diesel is more expensive but its not extremely expensive like stated about. They last longer with less major repairs. Yes those are really nice trucks! 2500 makes me wonder though.
Really, ok, let's do a side by side comparison. 5.7L Hemi and the 7.3L Powerstroke.
Glow plugs: $10 each for Motorcraft, need 8 because like spark plugs you should replace all together
Spark plugs: most expensive ones $13 each, standard are $2-3 each.
HPOP for 7.3 - $450 for a refurbished, best price I could find, a common failure on the 7.3
oil pump on 5.7L - $150
engine oil cooler for 7.3L - $150 + replacing all your coolant after it's fouled with oil, another common failure
5.7 doesn't have one
Oil change - $75 for a DIY (5 gallons of 15w40 runs around $65, plus $10 for the Motorcraft filter) - 7.3
5.7 - $40 for 7 qts of 5w20 plus filter - if you want to get fancy, $50 if you put full synthetic in
Fuel filter 7.3 - $30, change it every 15k miles or once a year
5.7 - $10
Fuel injectors, are right about $640 for stock on the 5.7L, cheapest I found for the diesel is $775, plus you end up replacing them at least once on the 7.3L because diesel is dirty and clogs them up. I'm looking at this at 197k miles, oh and that oil in the coolant from them failing, yeah
So basic maintenance on the 7.3L is $180 a year for 2 oil changes and a fuel filter.
The 5.7 is around $80 - $90 if you want to change the fuel filter, though recommended is every 30k miles.
And the labor costs if you can't DIY, because as my wallet is well aware of, diesel specific shops exist for a reason and they charge a lot, we do most DIY, but the HPOP was out of my league and was $300 in labor alone. Not to mention when it went, it dumped an entire gallon of oil out on the road.
Glow plugs will be the same, I can change out spark plugs in my driveway, but the glow plugs are under the cover and beyond my ability. Not all diesels have them, but mine does, so it will be a repair cost.
That said, diesel trucks are great, they will run forever, but owning 2 gas pickups and 1 diesel, the diesel costs me way more for parts and repairs than either of the other two, one being a 1984 which breaks down constantly.
The big upside is the fuel economy (even though diesel is a good 25-30 cents more per gallon here) I get about 15 MPG towing, which I don't get with the other two.
@misfitmorgan
I drive a 2000 F350, so basically the same thing as you're looking at. It will be a 7.3L Powerstroke, and at that price, something almost has to be wrong with it because they are worth more than that.
Towing wise? It'll tow about anything

You can kind of see from my list on basic things that go out on those. Check for oily residue in the radiator because that can be the injectors or oil cooler, and either way it sucks.
Also, see if it leaks and how bad. A small leak isn't a huge deal, a big leak can be your HPOP, and where the leak is coming from. If it's in the valley and soaked...yeah. Also, see how it starts when it is cold, and see if it's blowing smoke when it starts, a lot of white smoke is not a good thing.