On the page where you want to display amount value, create two text widgets, one is to display amount value other one is to display NA.
For the first one, add conditional visibility like if amount is > 0 then display amount text widget
For the second one, add conditional visibility like if amount = 0 then display NA text widget
Multiple ways to do this. Here is one: Place two containers on your page and set their visibility based on $YourDigit>0 and $YourDigit<=0. In the first container you add your digit. In the second container a textfield ‘N/A’.