Excel macro to multiply 2 columns (independant of number of rows)?

Hi,

I have excel sheets with sales data and I want to create a macro to multiply the last 2 columns together into a new column. Each row of the 2 columns is multiplied separately. Then sum all the numbers in the last (new) column.  I know how to multiply 2 columns in the same row and copy that for all rows but the rows will change from sheet to sheet. So the macro needs to know how many total rows there are. Here's an example

x  x   3   $1.00
x  x   5   $1.25
x  x  10   $0.75


so I want a macro to give me
x  x   3   $1.00  $3.00
x  x   5   $1.25  $6.25
x  x  10   $0.75  $7.50
                  $16.75



The macro should also work if there are 5 rows of data or 50 or 1 billion, etc. Any ideas would be greatly appreciated. Thanks!