Not a hypnobirther but I used to run antenatal courses. Most of the people doing hypnobirthing have it as an add-on to other stuff they're doing eg midwifery, doula-ing, standard antenatal classes, complementary therapies etc
If you don't need the money and are just doing it for the love of it that's great as it would be nigh on impossible to make a living from it. You have a smaller pool of potential clients to start with as the majority of expectant couples won't be interested so you may have to travel quite widely. Even then I'd be surprised if you could fill more than 2-3 classes a week (actually that would be very ambitious). Factor in the cost of venue hire, marketing, refreshments (you need good biscuits at antenatal classes!) and your admin costs and your hourly rate starts looking vanishingly small even before you recoup the cost of the course
I do a form of hypnobirthing and also antenatal classes. It is a nice additional income but not a career. I make far more from the antenatal teaching. There are lots of hypnobirthing practitioners to choose from ( the training is much much shorter and in some methods not vigerous IMHO). Also some local hospitals also offer classes which has a detrimental effect on bookings.