You can look at a financial investment as an alternative to DVC. Take the money you paid up front and invest it in a mutual fund or CDs or a money market account. Each year take money out of that account to pay for your trip to WDW and deposit into the account what your dues would have been if you bought into DVC. When the amount of money in the account hits zero you have hit what a lot of people consider their break-even point for buying into DVC.
Let's say for arguments sake I bought 160 points at BLT for $95 and I break even in 10 years. I paid my $15,000 in upfront money plus 10 years worth of dues and got 10 trips to WDW that would have cost me the same amount of money. If I sell 160 BLT points on the resale market at this point whatever I get paid is a gain for me. For arguments sake again, lets say BLT resale in 10 years is going for low $60s a point. After paying a sales commission I may net close to $9,000 if I sell. If you look at just the purchase and sale as a pure investment I bought in for $15,000 and I sold for $9,000 so I lost $6,000 on my investment. Where that logic is flawed is I got 10 trips to WDW for just my dues each year which is a significant savings over paying cash for a room. The better way to look at it is after breaking even whatever I sell my DVC for is my profit from the investment. If I don't sell ever and let the contract run its course whatever I save each year after break-even by paying dues instead of a cash room rate is my return. This all assumes you would actually be going to WDW either way.