You pay up front for the cup, depending on how many you want changes the cost. one cup is like 16 bucks, two is slightly cheaper (per cup) and 3-6 is a little cheaper again per cup. This is good for unlimited refills all day.
The cups have an RFID chip in the bottom, which is how the machines can tell if the cup is "active". Once your activation time is up you pay 8.99 plus tax to reactivate the cup for the day.
We found 2 cups worked well for our family of 4. You can get refills at both the coke station machines as well as the kiosks that have fountain access. The machines have a wide variety of options, the kiosks have a more limited menu. You can also get icees where available. I pre-bought 3 cups on Ebay then held out for the 4th prettier volcano bay cup only to find that they changed the style this year to one cup for all 3 parks. It does have a top handle which is nice. We activated the first one in VB and the lady either screwed up or it was late enough in the day we were able to use it for 2 days before it needed to be activated again.
I didn't pay attention to price point break, I knew we had invested in the cup and felt lucky to get it for 2 days the first time. It was stupid hot and humid there last week, so we really used ours for water more than anything else.