There are many ways to calculate "break even", some more sensible than others. A good analysis will include an annual percentage rate for financing, cost of annual dues, annual increases in dues and the cost of alternative accommodations, and a computation of remaining principal. If you are paying cash, a good way to handle it is as a "loan from yourself" with interest rate representing average investment gains (which you don't get since you spent the cash).
Even doing all this there are several alternatives.
(1) You can compare the cost of owning
DVC to the cost of renting points. Based on computations I did at the $75 level when I bought, I don't think you break even (ever) against renting at $10/pt if you buy at $90+/pt. But the cost of renting may increase unpredictibly and it will get harder and harder to rent at $10/pt. Also I think the lack of control and aggravation involving in renting do not make whatever savings result worthwhile.
(2) You can compare the cost of owning DVC to the cost of whatever alternative accommodations you were likely to use. This is a valid way to determine whether buying DVC actually saves YOU money, but it is comparing apples to oranges as the DVC rooms you get are likely to be much better than what you would have had otherwise. So DVC may not save money compared to Caribbean Beach but would almost certainly save money compared to Grand Floridian.
(3) You can compare the cost of owning DVC to the rack rate cost of DVC rooms. This is a "feel good" approach because buying definitely saves money from this perspective. But it's not a useful comparison because very few DVC members would have typically paid rack rates for DVC rooms on a regular basis, even assuming they were available.
(4) You can add up the total number of points you get throughout the remaining life of your contract, divide the total into your cost, and compare the resulting cost per point to renting. This is completely delusional from the point of view of standard accounting practices, even if you include annual dues somehow. But if it makes you happy, you're welcome to it.
And in all of this the type of room you get (studio, 1BR, 2BR) and the percentage of weekend nights plays a big role.
The bottom line I think:
If you're committed to at least annual trips to WDW, would like to have high-quality on-site accommodations, and are happy with the nature of DVC rooms, then the cost is reasonable, whether or not you officially "break even" at some point. But don't imagine that it's some sort of incredible bargain. Disney is not giving anything away here. They are making big bucks on DVC and are not afraid to charge what the market will bear.
And I agree with JimC. If you don't think you'll get your money's worth in 10-15 years, put your money somewhere else.
PS Did I mention I'm a very happy owner? I bought in 2002, when deluxe rooms could be had for a song and dance. (We're talking $149 for AKL savannah and $179 for Polynesian GV.) I had no idea how much deluxe room rates would increase in just 3-4 years as the discounts dried up.