Paula,
I find the value of DVC ownership to be hard to quantify, as everyone has a different way of calculating what it's worth to them. For me it was an emotional decision, but fortunately my husband is a finance man so he was able to do the math for us. I decided to consult my fellow moms on this question, and they brought up several excellent things to consider as well. Here are a few of our combined considerations:
Family size - If you have 3 or more children, you'll need 2 hotel rooms on every Disney vacation. The larger DVC villas hold great value for big families as you can use points for a larger suite and save money.
Frequency of travel - You said that you visit once per year, but would you like to come more often? When you do visit, how long do you stay?
Time of year - Do you usually travel in value or peak season? DVC holds value either way, but even more so if you visit during busier times of year (ie-school breaks)
Accommodation style - Where do you usually stay? If you are always at a value resort and you have no desire to upgrade that, then you might be right to pass on DVC. But if you want to stay at a moderate and especially in deluxe resorts, DVC will save you a lot of money.
I don't have enough space here to give you a complete breakdown, but basically I suggest you price out a vacation that would be typical for you, and compare that to the cost of DVC ownership. I'd assume at least a 3% inflation rate on that vacation every year. Compare that cost over 50 years to the price of DVC now, and the bottom line should give you your answer.
Mousesavers.com has an excellent breakdown of DVC cost comparison, and of course the
DVC website does as well.
Good luck!