You asked about group classes“. do you mean small group personal training?
Group ex is very different from personal training. In health clubs there tends to be an instructor rate, and then sometimes a slightly higher rate for specialty instructor. If your classes are consistently full sometimes such rates can be negotiated higher, though in big facilities, particularly chains, that may not be the case. As Karin said small ‘mom and pop’ facilities seem to pay better. Exactly what that pay is will differ depending on your geographic location.
The exception to this rule is in stand alone facilities. When I teach in a yoga studio if I sub I do get a straight rate (considerably higher than what I would earn in group ex in the same market), and if it is a regular class typically I get a percentage (typically 50%) of what the student has paid.
If you are teaching in a larger facility and they are charging extra for a specialty class sometimes you get a higher rate as well.