I think this can be done. Firslty, add a new coloumn called total. Then drag a datalabel to the coloumn. Then go to the expression property and create a expression where you take the two fileds and multiply it. THis will give you a total for each line. After teh calculation is done, click on the coloum you added and tick the sum check box which will automatically add  a total sum for that coloumn.

Hope that helps.

Just to add to the previous response, I think this may help you if you need some visuals. It basically outlines the same thing: http://help.k2.com/onlinehelp/k2smartforms/userguide/current/default.htm#using_expressions_on_an_editable_list_view.html

Thank you! that worked perfectly :)