Marcelo,
You could change your Default Currency Symbol to $ from the Customize > Display screen in Method. This would be system-wide, but it's the easiest way to get this done.
As far as the comma is concerned, it wouldn't be very straightforward. You could pull the value from the Balance field and assign it to an Action Result, then use the Character Function to manipulate it. You'd have to use Get Length and use a Basic Math Function to determine how many times to run a Get Right, separating the number in to Action Results of 3 characters each. You could then join them with commas and insert them into a Label on screen. You'd have to add a $ to the string as well in this case.
Hope this helps.
- Justin