Running a barn is definitely not a walk in the park and probably not your best option.
Your best option for your benefit in the horses, in my opinion :), is to talk to the BM. Make sure you land a spot in this potiental facility if it falls through (have options!) And let them know all of what you did and continue to do for the BM in exchange for lower fee. Be professional and borderline suck-up to enlighten her what you pay now and if it were at all possible to leave this unchanged.
Or you can have the current BM right up a contract saying that you pay $xxx amount of dollars per month and this is a contract legit regardless of the BM.