Insurance coverage for mental health ?

I have been diagnosed with a severe mental illness recently. I have been advised to purchase health insurance to help me pay for my doctor. I'd like to keep my doctor however, like many other psychotherapists, he is not the preferred provider of any insurance company. I'd like to know how much, in average, health insurance companies pay for mental health practitioners. I understand that my insurance plan will cover severe mental illness, but with non preferred doctors they will pay only a percentage. In average the percentage is around 50.
All suggestions are welcome!

Thanks