Since you can "rent" your points for $10 each or so, that is probably a good amount to use when deciding cash versus points. Take a one-bedroom at BCV during "Magic Season". It's 30 points on a weeknight, 60 on a weekend. Using your points on a weekend is like using something you could have "rented" for $600. But if you can get the room at Embassay Suites for $200 (including 11% taxes), it might be worth moving.
I've looked at the numbers from 75 different angles. One way I thought about it is that the resale for these is pretty strong...I wouldn't be surprised if you can resell for $70/point in 5-10 years. So lets assume that your points hold their value. What are you out? You have to pay the main. fees (say $4.10 on average over the next 5-10 years) and you either pay interest on the money you borrowed to buy
DVC or, if you had the cash available, you've given up earning interest by investing that money somewhere else. The bast rate you can borrow money through Disney is 10.95%, but this is tax-deductible. A little over 7% after-taxes for most DVC buyers I would guess. That's probably a pretty-good after-tax "what I could have earned on my money" amount as well.
So....$70/point * 7% per year = $4.90.
Main. fees of $4.10.
You're at about $9/point average cost to you making all of those assumptions.
So....the one-bedroom during "Magic Season" goes for 60 points on the weekend. Is it worth $540/night to you to stay in the one-bedroom on Friday or Saturday night? Because if you buy 60 extra points to have enough points to cover that room for that night, that's pretty close to what your annual cost of getting that night is worth if those assumptions are correct for you.
If you're borrowing money at 4% after taxes (e.g. 6% home equity loan), then the longer-term cost per point might be more like $7/point ($420/year to buy 60 extra points). But if you're putting it on a charge card and won't pay it off for years, the cost is much higher.
I've also looked at this over the full forty years, and come to a pretty similar conclusion.
Hope this helps!