We put all of the costs into a spreadsheet, with multiple point values, cash options and different resorts. With a family of 6 (4 kids), it eliminated budget hotels and such both on and off property, and the fact that we could only stay in the summer (due to our jobs) really simplified the spreadsheet. We didn't have to take various times of year into account - just summer.
We know that we travel about every other year, maybe a little more frequent as the kids get older. I think that was a key point for us in determining how many points we needed to really use. We found that there was a whole range of options from about 200-380 points that financially made more sense than paying cash. We found that the break even came after 7.5 visits.
One of the small items that most don't take into account (we didn't either at the time) is food. In a budget room, you don't usually have a kitchen to cook with. We save a lot money on our trips by doing most breakfasts in the room (still have to go out occasionally, it is vacation), and usually do a few lunches and dinners in the room. The savings for a family of 6 with that is staggering. It was not uncommon for us to spend over $50 for breakfast and closer to $100 for other meals in the parts. We figure that we "saving" over $500 dollars per visit by being in a DVC room with a kitchen.
The investment piece is one that can't really be figured in easily. Everybody invests differently. When we bought (5 years ago), if I had figured a 5-7% return on the money, I would have shockingly wrong. That money would have been worth 50% less today then it was then. Could it go back up, sure, but the last few years have shown me that investment is not as straight-forward a proposition. Over 50 years it is more clear, but there is still a lot of unknowns.
In the end, DVC is a luxury purchase, not an investment. You are borrowing from your future vacation/discretionary income, not money that you would have invested in savings. You can't really pool the two together and consider them the same thing since you spend them differently. I understand that you "could have" invested and then pulled it out with it being worth more, but that is not how most of us deal with luxury purchases.
Create a spreadsheet comparing the real costs of the room only (leave the other benefits out). Compare the room level that you have used for your last few vacations.
For us, purchasing direct through Disney broke even after 17 visits and gave us other benefits. In the end, we bought resale because we weren't sure we would vacation the same way once the kids grew up, so we may not need nearly as many points when they are gone, so we bought resale and gave up some flexibility for destinations to be sure that it financially made sense.
It already made sense in the heart, that's why we looked at DVC in the first place.