They are free when you collect your baggage from the belts before passing through customs. Once through customs you then have to return that and if you can't manage the luggage without one you will have to place it on the belt to be taken to the main terminal for you.
Once at the main terminal there are cart that accept cash or card and you pay an amount and get some back when you return them, otherwise there are skycaps that will help you with the luggage who works for tips. When DD was young we generally used a skycap whilst it was a couple of dollars more it was nice just to have someone look after the luggage and load it for us!