need help with a button that would delete an amount, then to show it again?

Hello all, what I need with is a javascript button that will enable me to click it and the result will remove the amount from the calculation, click it again and the amount that was removed will be added again.

Let me explain what I'm trying to do. I am creating a form that shows the total of material costs, then adds a percentage markup to the material costs from a drop down. Then I have a field to select the labor for the job. I'd like to have a button to remove the labor to see only the material costs by itself. Then click the button again and the labor total would be added once again without having to re-type it in. Would someone have a javascript that would help me do what I want? I hope it can be done. Thanks.