Use the field formulas
Assuming that you are using Word 2003, place the cursor in the table cell for the first hours x rate calculation
From the menu bar select Insert and Field...
Select =(Formula) from the field names: list
On the right click the Formula... button
Replace the default formula with something like this
=A2*B2where A2 and B2 are table cell references.
(You can add number formatting later)
If you have some values in a column using the above formula, when you insert a formula field below them, Word will offer a default formula =SUM(ABOVE)
Number formating can be added by selecting the field and right-click and select 'Edit field' and click the formula button and enter a number format (there is a drop-down list for standard formats or enter your own).
If you select a field and right-click and select 'Toggle Field Codes' the number format will appear after the formula as something like this \#"00.00"