just think of all the sports stats most guys hold in their heads - worthless most of the time but useful for having a conversation with other guys...
But really, being a finance wonk by training, nothing convinced me faster than running the numbers. Doing my present value calculation, my breakeven period came out to about 5 yrs if paying cash, about 7-8 yrs with financing (depending on term). That was compared to the Moderate resorts or the deluxe resorts on sale (which as a cheap Mainer was about as expensive as I would allow most years). In our case, we have a DS who is wheelchair bound, which makes WDW the most convenient vacation spot to go (warm and totally handicap freindly), and means that we're going once a year in all likelihood. Notably, with banking and borrowing, all you really need to break even is to go once every three years (so you don't have points expire unused). An added advantage is that you can save on Park Admission by doing as so many DVC members do, and buying APs on one visit and doing next year's visit 11 months later (or within the same 12 month period); that savings takes a nice chunk out of annual maintenence fees, if you are doing an honest accounting.
Let him run the numbers, if he is a numbers guy. Just make sure he does an advantageous comparison, like comparing to rack rate at the deluxe resorts, if you want to be persuasive. As a discount rate, I figure the real cost of money in most households is the home mortgage rate.
Good luck with your persuading.
I just bought my wife flowers, candy, and a baloon. I hope I am not in the dog house. 
