Apple Numbers '08: extending a formula
Note: This works with all versions of Numbers.

To create a running total, if that is what you are asking, use two columns.  One for numbers to subtract, the other for the running total.
Add another column for numbers to add if desired.

In the example below, the starting number is in cell reference B2 and the number to subtract from it is in cell reference A3.  Cell B3 enter this formula: =B2 - A3 then, click and drag the yellow dot (shown in the illustration) to autofill the formula down into the cells below.  Numbers will update the cell references automatically.  

Of course, this can be improved upon but this is the basic formula to answer the OP's question.