This also assumes you are talking room only. It ignores risk - the risk that your vacation habits will change (with kids it isn't uncommon for teenagers to get bored of Disney - there aren't a lot of big rides - my son prefers a beach, my daughter museums - they are teens). Or that if you didn't own you'd skip years or save money one year with an offsite stay. It doesn't account for any deals you might get - like "free dining" - which wouldn't be available when staying on points. It doesn't include resale value - if you can sell in year 8 for $20k, you'll have come out ahead. It doesn't include psychological factors or changes to habits "we don't have a room bill, lets book a fireworks cruise" or "we have a kitchen, lets eat in the room." Possibly most importantly, its room only - since we've owned, tickets, food and airfare have all gotten much more expensive - we've locked in our room costs (more or less, dues go up, but not much), but we can't control airfare and park ticket costs - and once we had four Disney adults, park ticket costs became a much bigger deal.
My own belief is that if you need the math to work financially, DVC isn't for you. Look at the offering - warts and all - and decide if its a good VALUE - if you want the timeshare experience over the resort experience. And if you are likely to continue to want that for five to seven years - beyond that Disney and life changes too much to see very clearly. Then ask if you can afford to "lose" your "investment" in DVC. If you think you want the DVC experience for the next five years or so, and you can afford the risk - go for it.