When we decided to buy, we looked at what we were paying for just a hotel room at the CR and what we could get by buying DVC. So, it wasn't so much about paying less for lodging, but rather getting bigger and better rooms for no more than we were paying. We assumed a 30% discount as our base.
What we realized that to buy BLT, it would take 10 years to break "even" on the initial cost of those points. We assumed a resale value of 0 so once we pass that threshold, anything we get will be a welcome surprise.
We also looked at worst case scenario and having to sell points prior to the 10 year mark. If that happens, then it would mean we would have spent a bit more than what we could have had we stayed cash guests. In the end, we decided that it was worth the risk and bought.
Now, in terms of us actually saving us money--nope, not a chance. We used to go once a year, in the summer, for 5 or 6 nights. In 2012, I went 4 times. However, my trips now are with lots of family and friends. We stay in 1 and 2 bedrooms and get to Disney a lot more than we did before. So, from a strictly financial standpoint, my vacation budget has not gone down at all. But, I am getting SO much more for that money--bigger rooms, and a lot more fun at WDW!!
Basically, what I am saving on lodging is allowing me to go more often and have more memories with my family....something that would just not have happened if I didn't own DVC.
And, since I just put in offer for another 100 BLT points, I'm DEFINITELY tipping the scales in it costing me more!!!!