When we were there in late August, it was $10 for the entire day. Other DISers have reported that later in the day, you may find abandoned tubes lying around that you can grab for free.
We had no problems using our own inflatable noodles and tubes.
Also, I went in without a tube and was fine, just floating on my back with the currant pushing me along.
Remember to be careful when you step in though, it is quite a drop off, something like six feet.