Soda is around $2 each time. 12 oz. sounds about right as far as the size of the refillable mug. You can also fill the mug with hot chocolate, tea, or nescafe/coffee in addition to soda.
We stayed at POFQ where the one bus stop is right next to the lobby/food court. We found it very convenient to fill up the mugs before getting on the bus and then refill them when we came back to the resort. DH and I each got our own mug and we shared with our two young children.
Also, the slender version on the mug has a top that can be rotated to "close" or "open" the mug. We didn't received any complaints from WDW bus drivers when carrying these mugs on the busses even though signs indicated "No food or drink". We just took the mugs on in their "closed" position and our warm morning drinks were still comfortably warm when we reached the parks.