I know you guys have long moved past this point, but I just wanted to go back to something that I think is worth noting.
This is your fatal flaw, both from a logical and accounting perspective. Yes, the money has been paid up front, but it has been paid. It cannot be ignored.
It seems to me you are talking about analyzing a different question from the one iluvthsgam is talking about.
Question 1: "Is DVC a good purchase, financially"
Question 2: "Given that I have bought into DVC, does it make sense for me to spend my points on a cruise?"
The answer to #1 absolutely should take into account the buy-in, and you should account for the buy-in using an amortization calculation with interest rather than a straight division of the purchase price by the number of years remaining.
The answer to #2 has to take into account the sunk cost, or rather not factor it in, because it's sunk. Not treating a sunk cost as sunk is just as much a financial mistake as ignoring buy-in when evaluating a timeshare.
In fact, because your contract has a resale value, the entire buy-in is not sunk. Only the instant depreciation that happened when you bought direct. So if you have not yet bought DVC, you should treat the purchase price (amortized) as a real cost that needs to be added to your dues to calculate a "real" annual cost.
After you purchase DVC, you have lost forever about 30-40% of your purchase price, and that's sunk now, like the Lusitania. It's never coming back. But you own an asset that can be converted to cash, and that is something that can and should be amortized into your ongoing costs, because at any point you could sell it and buy a bond with the money, so by continuing to own it you're basically forgoing an income stream.
So you're both wrong. Ha!
I kid, I kid.
Anyway, to give an example:
Joe Doaks is thinking about buying BLT direct. It's $165, and the dues are $4.50. Long-term interest on safe bond funds is 4.5%. What is his amortized cost per year of ownership?
In Excel, you use the PMT function to calculate amortized value of a lump-sum purchase that pays off over time. So PMT(4.5%, $165, 47) = $8.50. Add $4.50 dues and you have Joe's expected annual cost of $13 per point, which of course will go up as dues go up.
OK, so Joe says, that's good, I'll buy. Now the odd thing is that he should call the difference between the direct price and the realizable resale price a sunk cost. So he just paid a lump sum of let's say $65 per point to get the right to use his points for various financially disadvantageous exchanges. That $65 cannot be recouped, so it's just gone. Now his cost per year has gone down, because it's now PMT(4.5%, $100, 47) + $4.50. That's $5.15 + $4.50 = $9.65. Once he's paid the sunk costs, they're sunk. Can't keep kicking himself over having paid them. Though he should, because he should have seen them coming.
That's why it can both be true that buying a DVC membership primarily to go on cruises is a terrible, awful, horrible decision, and yet it can make perfect financial sense to rent points to buy a cruise once you've already bought into DVC, even factoring in the amortized value of the property itself.
The bottom line with all this is that the "true" value of DVC points is the resale value, because that's the only value that's set in a competitive marketplace. Just like the "true" value of a car is the resale price. The amount you pay over the "true" price is just money thrown away. Everyone understands this perfectly well with houses, but somehow people can't quite wrap their heads around it when it comes to cars and timeshares.