Form Calculation: How to use a comma to separate thousands?

How do you add a comma as a separator so that when an amount is in the thousands, it appears as  3,249 and not 3249?

You could use the commaSeparate() function to achieve it, to use it simply click on the options button in the widget wizard: 

On the math functions tab you need to scroll down until find the function: 

Do note that all the calculations need to be done between the function parenthesis, the result should be like this: 

Hope this helps.