It is hard to put it into a tangible explanation that an accountant type would easily get - I know - I am one

.
IMHO, there is just something different about staying "on property". We have stayed both on and off property, and we find the difference obvious.
When you stay off property, you go to the parks and enter the Magical world of Disney. Then you leave the parks and go back to your non-Disney hotel, and feel like you are leaving Disney and going someplace else. Yes, your hotel might be nice, but it is just a hotel. Yes, it might be close to Disney, but it isn't Disney.
When you stay on site, after you leave the parks you never leave the Magical world of Disney. The on property resorts are kind of an extension of the parks. You never feel like you are leaving Disney, you are just going someplace different within Disney. That Magical feeling you get is with you 24/7 during your vacation.
When we stay at a Disney resort we feel like we are vacation in WDW. When we stay off site we feel like we are vacationing in Orlando and visiting the WDW parks. A subtle difference, but one you probably need to experience to know if it will make a difference to you.
Let me try it this way. We have stayed off site at the Courtyard in Downtown Disney. When we leave the parks and go back to the hotel, unless we leave the hotel the evening is over. The hotel is nice, and there is a pool, but there isn't much to do. You can't walk around the hotel property and soak in atmosphere. You can't do much. Contrast that with staying at an Epcot resort for example. You go back to your hotel and you have a world of options available to you. You could walk around your resort as the grounds will be beautiful and rather extensive. The pools will be incredible. You have many restaurants to choose from - all within walking distance. You could walk over to the Boardwalk or check out the other Epcot resorts. All this while you are still immersed in the Magic of being IN Disney. Even if you stay at a moderate or the values, the resort grounds are much more extensive than a typical off site hotel and the themeing blows off site hotels away.
The difficult thing in all this is putting a value on the feeling you get, the extension of the Disney magic throughout every waking minute of your vacation. That is the hard part for the accountant. That is where the tangible benefits come into play. Ultimate park hopper passes that are good for your length of stay might provide you a dollars and cents benefit. Being able to take advantage of the Extra Magic Hour daily at one of the parks is worth something as well. Perhaps being able to take advantage of an E-night if there is one.
See if you can get him to agree just once - and expeince the difference. Maybe you don't notice one and you find you were just as happy off site, or maybe you do and your already wonderful vacations get even better. Just to find out is worth the investment for one trip.