Is this a fair percentage/monthly rent?
I believe I've read somewhere on here, I think from R&R, that a business cannot afford to pay their therapists more than 40% of the treatment retail price. For myself, having run a spa for the first time I am now seeing this to be very true. I am paying 50% because that is the "going rate" in this town (it's actually on the low end, some places pay more) And we have to be doing a good volume in order to make any money at all.
In your example you'd be paying them 70% and charging a flat monthly fee to cover expenses. If you are really providing EVERYTHING I doubt this arrangement will work out for you. However, if they are taking care of their own booking, if there is no receptionist, and if you are not doing marketing on a regular basis then perhaps it may work. Those are my biggest expenses that are there every month whether there are clients booking or not.
Have you calculated every single one of your expenses? Not only the ones that are there when clients come, but the expenses that are there no matter what? Once I did a spreadsheet with actual numbers everything became much clearer. You can do a spreadsheet where you change the number of clients you see and the % of payment and see how the numbers come out.