Modifying Pivot table cell

Hello,

I am using a pivot table to figure costings for construction. I have the following two columns:

Item, Unit Cost

What I would like to do is be able to add in an additional Quantity column and be able to modify this value on the fly so that I can calculate the total cost without modifying the original table. The Pivot table would look something like this:

Item, Unit Cost, Quantity, Total

where Total = Unit Cost * Quantity.

Whenever I try to modify a value in the Quantity cell it tells me take I can't perform that operation. Is there any possible way to accomplish this? Would this require programming in VB? If so, can someone please point me in the right direction?

Thank you,

-Travis