If you are on a dining plan you would not have to pay for them at all. You would receive "mug credit" that is encoded on your KTTW card just like your dining credits. Pick up your mugs and use the KTTW card to pay for them.
The RFID dispensers are being installed and are currently not operational - they also couldn't say when they might be operational. So we don't know how they become activated. But the dining plan includes refills for the length of your stay (I do remember that part of what the dispensers do is regulate the number of refills you can get within a certain time - so it's possible you can't get as many as you might want without waiting a bit)