I don't calculate it. Because I don't save money.
DVC has changed how we vacation at Disney. We wouldn't have paid for rooms for friends and family without DVC. We wouldn't stay in a two bedroom unit for the four of us without DVC - we'd be in a hotel room. We wouldn't go as often.
So any comparison we'd do is apples and oranges.
If you take pretty much the exact same trips that you'd take without DVC - you stayed in a regular hotel room before, you stay in a studio now. Or you rented points and now use your own points. You go the same amount. The lack of hotel bill and frequent trips don't encourage activities you wouldn't have done. You don't take guests - then you can compare and figure out how much you save. I think there have been half a dozen people on this board that say that describes them. For most of us, its at best a fun exercise in math and accounting, and at worse, a justification to overspend.