You can refill the mugs with whatever beverage they have on the self-serve drink island/area. So that pretty much means, soda, tea, coffee and hot cocoa. Occaisionally you can find juices, usually apple juice.
They are usually $12-15 a piece in that ballpark number. They are worth it if you are not staying on the concierge floor since you get free drinks there but otherwise they are worth it.