I would suggest eliminating the 30 min option. A group needs setup and tear down time.
Even 45 min may be unworkable depending upon the size of the group. So perhaps figure on 1 hr.
Conduct a survey in your town and the surrounding areas. What is the going hourly rate?
In these other settings, How many lessons are students required to take in order to register for a group class? (Allowing people to float in and out on a week to week basis, could get counterproductive and cumbersome).
After that, determine what you may offer to students that the other groups may not, i.e. Specializing in the very young, the adult learner, a specific style, ensemble, studio class, open mike or recital opportunities. What is the added value of said options?
Be certain to figure in your actual expenses: travel, room rental, student instructional materials etc. Add it all up along with any subsidy available from recreation dept, educational institution, arts& humanities grants or generous private benefactors (just kidding), then divide total by minimum number of students anticipated to enroll. If more students enroll than you anticipated, congratulations on a modest profit.
I taught adults and children guitar for about a year in separate groups. Fortunately, a college was running the program, so the administrative burdens of advertising, enrollment, collecting fees etc. did not fall on my shoulders. Even so, it was a lot of responsibility involving a lot of preparation, planning and energy. I enjoyed it, but I decided that the payoff (for a part time job) was insufficient after taxes.
Good Luck and all the best!