How to calculate the cumulative balance of an account over time

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Here's what I'm trying to do, and I just can't seem to find a formula that will work.  Any help you could give would be greatly appreciated.

The scenario:

You are paying rent based on the space that your inventory takes up in a warehouse.  You know what your first month's rent is, and how many units were in the warehouse that month (and, consequently, what your per unit rent is).  You also know how many units you sell per month.  So how would you calculate the total amount of rent you will pay on the units you have between now and the time you sell out?  And then how would you calculate the total amount of rent you will pay over time on the units you have if you were to sell 10% more units per month?  And 20% more per month?  You would not only be shortening the amount of months which you pay rent, but also decreasing the amount of rent you pay each month at a faster rate.

The easier to understand hypothetical scenario for this problem is this:

What if I had $10,000 in a bank account, and withdrew $10 per month?  What would be the grand total of your monthly balances over time until you withdraw all $10,000?  Now what would it be if you withdrew $11 a month?  And how about if you withdrew $12 a month?  And so on, and so on.....

What we are really trying to do here is determine would it make more sense to mark down a product and pay less rent, or sell at full retail and pay more rent for a longer time?

Thanks for your help!

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