It's fairly standard for many suboxone doctors to not accept insurance. Many of the private doctors in my area charge cash for office visits. Insurance then generally covers the cost of the prescription with a minimal co-pay. There are a few public clinics that offer suboxone treatment in my area that do accept insurance, but they generally have very long waiting lists, ranging from 1 month to 6 months+. These public clinics can only accept a certain number of patients by law. I'm fairly sure that the same rings true for private doctors and psychiatrists, but they're more lenient as the appointments tend to be very expensive. When I was on suboxone, my initial appointment to see the doctor cost $250, and it was $150 a month every month thereafter. The prescription was fortunately covered by insurance with a small co-pay and for that I am grateful.
I would look at it this way - how much are you spending on your DOC each week/month? I'm sure $300 is minimal compared to the amount you're spending on drugs. $300 to get better is very reasonable, in my opinion, especially since your mother is willing to help cover the costs.
Best of luck!