From Number Format drop down list select one which includes dollar sign as we are dealing with currency values click OK to continue. The Table Tools tabs become available. In the Formula box check the text between the parentheses to make sure Word includes the cells you want to sum and click OK.
The process is as easy as creating an expression field and adding the formula field but somewhat hidden from the normal options Word gives you for using the mail merge fields.
On the Layout tab under Table Tools click Formula. It means Word will sum all numbers in the left of R2C4. On the Layout tab under Table Tools click Formula. Place the cursor at the blank cell of the first row then click layout Formula see screenshot.