I’ve never noticed a higher up keep cost on my diesel vehicles versus my gas vehicles.
Depends on how you figure it. As PF stated, the 7.3 Fords (I used to own one) took 15 quarts of oil, my 6.7 Cummins only takes 12. On the plus side I only change it every 7500 miles, to be fair I only change the synthetic oil in my newer Tacoma every 7500 also, but every previous gas rig I've changed the oil every 3000. If you have any work done on a diesel at a shop, it costs a lot more than the same work done on a gas rig. On the plus side I know a lot of guys getting 500-600,000 miles out of their Cummins without a rebuild. By that time the rest of the truck has pretty much fallen apart around the motor and they usually replace the truck even if the motor is stilling running fine. Not many gas rigs you'll do that with, and none that I know of with comparable pulling and hauling power get that kind of longevity. Doing work yourself it is a crapshoot whether parts are more expensive or comparable to a gas rig, depends on the part. Transmissions built for diesels (some run the same tranny as their gas counterpart, but tend to have more issues because of the added torque) tend to be much more expensive to work on for example.
Good diesels (there are some junk diesel motors out there that are constantly broken down, I'm not naming names though, I'll let someone else do that

)tend to get more miles with less up keep, but the up keep is more expensive. They get better fuel mileage, but since the government has become anti diesel the fuel costs more. All those costs end up probably being close to a wash, with the edge going to diesels I figure. But the initial cost of buying a diesel is higher (much higher on the used market, they hold their value much better). So unless you have an actual use and reason to own a diesel it may be cheaper in the long run to buy a gas. Big diesel trucks have become a fad, which is annoying but actually great for those of us who actually have a use for one and don't have/don't want to spend the money on a new one. There are lots of gently used (read commuter vehicles who have never seen a trailer or heavy load in their life) diesels out there for sale, used to be the only diesels were work rigs, and when they came up for sale used they had been rode hard and put away wet.