DCL has gone a long way to "clean up" the hair braiding on the islands, working thru the local authorities. You can have it done on the ship, on Castaway, or at any of the ports of call. The costs are supposed to be posted, and they are supposed to stick to the posted cost (a big change from 10 years ago!)
8 years ago, on another line, DD had one braid done. The sign said "$2 per braid." When she finished, the woman wanted $10, claiming that the extra was for "long hair" and "beads." The hair was above shoulder length, and she used 4 plastic beads that I can get at any craft store for 1 cent each (trust me, there are beads like this all over my house!). I gave her $5 and walked away. This stuff doesn't happen any more since DCL "got tough" with local authorities.
The signs now say things like $2 per braid, long hair 50 cents extra, plus cost of beads. That makes it clear that you need to negotiate the cost of the beads ahead of time...they should charge you about 10 cents per bead. That adds up to about $3 for a braid with long hair.
There is room to negotiate if you want a whole head done, Get a price for the head in advance, then stick to it with a small tip!