My DD got hers done outside the shop at Pirates of the Carribean, Magic Kingdom. Her hair is shoulder length and it was about $30 for 2/3 colours with a few beads.
My daughter got one done on the Boardwalk and another done at Animal Kingdom. If you have them wrap from under the hair (behind the ear) and not from the top it's cheaper. My DD's long hair was about $30 with beads and it lasted months.
If its wraps you're talking about rather than braids then I've seen them done at SeaWorld, Carribean Beach Resort & Typhoon Lagoon in the past for $1.50 an inch plus the cost of the beads. Last two trips my DDs have had a wrap done by a lovely lady at the Premium Outlets for $1 an inch (4 colours and she does some lovely patterns as well). I cost just under $40 for both of them - one has long hair and the other a shoulder length bob if that's any help