I just went through an exercise like that and discovered that it's more or less a wash for me.
Base assumption, 2 adults and 2 kids arriving Dec. 11, 04 and figures same transportation expenses to either destination.
For
DCL, I assumed a cat 9 room on DCL, traveling off-season, basically no excursions (because I don't do 'em), minimum suggested tip, +$50 for additional tips, $50 for Palo, $200 for souvenirs, ~$150 for insurance, and my $200 onboard credit I currently have. Works out to $532/nights for all 4 of us.
For WDW, I assumed $180/day for tickets, $100/night for onsite moderate room, $150/day for food, $30/day for souvenirs, or $460/night for all 4 of us.
A few observations: at $150/day at WDW, the quality of my meals would be somewhat less than the quality I'd experience on DCL, when I go to WDW I never stay for 7 nights (typically 5), so overall expense would be less, but I tend to spend more on souvenirs and extra activities at WDW that I wouldn't spend on DCL.
Bottom line - they're different and each has its merits. But right now, I'm in the DCL camp because I find a cruise to be far more relaxing than WDW which, for some reason, I generally find stressful.
Sam