A flat, hourly rate is nice; and rates do depend on the location where you live.
However, the best way to work with a fitness center is to get a percentage of the class fees.
Generally speaking, health clubs try to get 50%, so they want a 50/50 split.
Some Yoga teachers are getting 60% and even more.
Splitting up the class fees between the Yoga teacher and health club is a win-win situation for both parties. The reason is because Yoga teachers will market their classes with flyers and other methods of advertising, including using the web.
A health club could not get a better deal – and your class fees, per hour, could be huge.