Last we were there (2 yrs ago), it was $1 per inch, plus beads (started at 75 cents each). Last year at US, it was $1.25 per inch, so I'd tend to think WDW has gone up as well.
To keep the cost down, you can start from the hair behind the ear, rather than on top of the hear (shorter distance).
I ususally pay $30-$35 for my dd's shoulder lenght hair.