There is a concept in Finance called Time Value of Money. Basically, it means that a dollar today buys more than a dollar tomorrow. This is because as time go's by, costs of goods and services generally go up. For example, a chocolate bar used to cost 5 cents. Now, can you even get one for a dollar? Chocolate bars don't all of a sudden eat up 20x more of your budget. Its because the avg income has gone up by 20x as well.
The same concept works for
DVC. Average US inflation is about 2% per year. This means that the average good and service goes up by about 2% every year. So if maintenance fees go up by 2% next year, but your income also goes up by 2%, theres really no effect to you. However, if maintnenace fees go up by 3%, its now eating a larger portion of your disposable income. But not not 3% more. Only 1% more.
So in order to make your projections meaningful, you have to "discount" the future year's cash flows (difference between rack rate and maintenance fees) into "todays" dollars.
The formula to do this is:
C/(1 + r)^n
C = periods cash flow
R = inflation rate (2% is conventional to use)
N = period number after start period. So if its 2019 today, in 2021 use 2. 2019 = 0, 2020 = 1, 2021 = 2, etc....
If you do this in excel, the formula is =PV(0.02,N,0,-C,0).
Sorry its hard to explain in a post. But it is important for the calculations to actually make sense