This is a great question. I don’t know the statistics, but anecdotally I know of several costume design grads who are working in their field. One of them, according to a former professor, is making more than he is. It is my understanding from my daughter’s interviews that there is some crossover between film and theatre, but I don’t know anyone personally who has done that. My D would like to work in both. The program she is starting in the fall appealed to her partially because they have an LA campus where students can spend a semester or a summer.
I also think that many costume designers (maybe the most successful?) work freelance or own their own businesses eventually, so maybe their higher salaries are not reflected at Payscale. Just a thought.
Salary aside, it seems to me that there are many many community and regional theatres that need costumers, so it seems that work would be plentiful, even if money is not.