It’s all about demand and ability, as are most jobs in the movie industry. There’s a lot of money in the business, and if you can prove that you do excellent work and exhibit some staying power, you can make quite a nice living at this. If you specialize in only one or the other, you may make a bit less, unless you’re doing your specializing on the sets of big-budget blockbusters.
If you’re working mainly on films, that means you’re bouncing from one job to the next – you may have some security if particular directors love you and keep hiring you on, but it might be less of a headache to work for a long-running television program, or to be salaried by a network to perform your magic on multiple shows. You should probably be able to bring in 40-50k a year; more if you have an established career in the industry and work is very regular, less if you’re working in theatre. And don’t forget that you’ll have the chance to travel to interesting or exotic locations for shoots, with travel expenses paid. Maybe even first class, if you do enough kissing up to the director.
Okay... maybe not first class on the Concorde.
If you’re applying your skills to theatre, or are stationed somewhere other than in the heart of Hollywood, you’re bound to struggle to make ends meet. And you have enough trouble trying to get split ends to meet.