Adding and subtracting variables

In my attached file, I have indicated the column containing the variables that I am adding to the "Total Spent" at the bottom. When the drop down selection changes, the addition works as expected. However, when I change any of my selections to lesser amounts, how would I set up the triggers so the Total Spent decreases? Is this even possible with drop-down menus?

Any insight is appreciated. I was able to achieve a lot of the conditions I wanted for this activity, but can't seem to get this to work.