Pixieduster:
No problemo.
I used a financial calculator to arrive at the result.
Let's do a hypothetical first. If someone offered you a choice of $100,000 now or $5,000 per year for the next 20 years, which would you choose? You would obviously choose the $100,000. Why? Because you can get much more than $5,000 per year for 20 years with $100,000. You can invest the money and earn interest on it. Considering the interst you would earn, you could withdraw roughly $9000 per year over 20 years (instead of getting only $5000 per year over 20 years).
That's why we can't just take $25,000 and divide by 40 to get (approximately) $600 per year. Once we figure the interest we would have earned on that money, we would have been able to withdraw about $1700 per year to have the account run dry after 40 years.
It's the time value of money. You are paying up front for a vacation that you will not receive (on average) until 20 years later. Of course, the vacations you will receive next year are worth a lot more than the vacation you will receive 40 years from now.
Here is the approximate break down.
Received in year 1 = 6.8% of value.
Year 2 = 6.34% of value.
Year 3 = 5.91% of value.
Year 4 = 5.50% of value.
Year 5 = 5.13% of value.
Year 10 = 3.36% of value.
Year 20 = 1.66% of value.
Year 30 = 0.82% of value.
Year 40 = 0.41% of value.
What does this mean? Well, of the $25,000, you would be paying 0.41% of that for the value you receive in year 40. You are paying $102 for the vacation you will receive 40 years from now.
You are paying $415 now for a vacation you will receive 20 years from now.
You are paying $840 now for a vacation you will receive 10 years from now.
You are paying $1700 of that $25,000 for the vacation you receive in the first year.
You are paying $1477 of that $25,000 for the vacation you will receive in the third year.
I hope this helps to clarify just what you are paying for and how it breaks down. I find it fascinating to see these breakdowns. Isn't this a fun chart?