We used to consistently spend at least $600 a night when we travelled with my DM, as we needed either a suite or two bedrooms. We always stayed at Deluxes, and since it became available, we always stayed at Club level. We purchased DVC about a year ago. If we had not purchased, we would have made two trips between the second half of 2008 and the end of 2009. (We did take a third trip in this period because we had the extra points and we'd bought APs, so we could do it for the price of transportation and food.)
Based on what we'd spent before, not on the actual price of the DVC rooms, we'd have spent ~$7200 ($600 per night, 12 nights) before fees and taxes. We have to have an internet connection at all times, so that perk saved us another $120 if we'd had one room, $240 if we'd had two rooms. The DVC golf membership saved my DM and her DH close to $1000 in greens fees, which they would have paid, regardless of DVC membership, as well. We also saved $300 on our APs. So, even very conservatively, we would have spent at least $8600 on room and perks in this year and a half period.
Plus, we got to bank points and get a bonus trip this year, because of developer points and the banked points on the resale contract we purchased. Now, admittedly, without having bought DVC, this additional trip would probably have been a cruise. We would have taken some vacation, but it probably wouldn't have been WDW. This trip was just me, DH, and DS, so we just needed one room. If we'd booked it at WDW, we'd have spent another $1800 on the room. (6 nights at $300 a night.) We actually decided that even with the great cruise deals offered this year, we weren't going to beat just the cost of food and transportation; the DVC, AP, and TiW card combined to make WDW the cheapest vacation we could take. Since we would have taken some trip, I feel pretty comfortable adding the room value to our total for what we would have spent on vacations in this 18 month period. (We'd have spent more than that on a cruise.)
That brings our total of actual value to us to at least $10400, although, of course, we'd have spent more than that if we'd paid for the actual rooms we had in this period. We won't have that many points any other year, since we had developer points and banked points from our purchases, so I suspect our real break even point will not come until year 7, or so. It will take a little longer if they eliminate some of the perks (namely, discounted APs, free internet, and the golf membership.) Even so, that is pretty impressive.
As for, do we really want to vacation at WDW for the next 40 years... well, I've wanted to for the first 40 years, I don't know why that would change!
(Ok, I'm not quite 40, but even so...)