The sign up location and time varies a lot. When we did it, sign up was day 2 by the goofy pool. I believe the cost is $1 for regular braid, $2 for cornrow. There's no difference in cost between full cornrow or "headband" style cornrow, but they ask that you tell them which one you want, since some of the braiders can't do the headband style.
I don't think they're really giving you the run around, but it changes enough that there's no standard answer.