Can't Clear Textfield / Resolve This?

I am fairly new to javascript and am having trouble clearing a text field. I am making a "vending machine simulator" as an exercise. I have a button that is the coin return, and should clear the balance display. e.g the machine shows a balance of $2 inserted, the coin return button should clear it to 0.

I believe I have done everything correctly but get NaN as the output (a text field named "box1"). I believe it has something to do with how the field is validated. It has number precision set to 2, which I would think would just become 0.00 or something. Anyways, here is my code code...