Hi Thomas,
Welcome to MyMedicareForum.com!
Your question is a tough one and I don't think you will find the answer online anywhere. Medicare has contracts with insurance companies to offer the Medicare Advantage Plans (Part C). Each insurance company offers various services for various costs to the consumer. Some include prescription drug coverage, and some do not. While you can find out how much the premium is for these plans by visiting Medicare.gov, you will find that the premiums are all over the spectrum. It's like asking "what does a typical car cost".
I know that you asked how much does Medicare pay the insurers, and I think you will have even less success in finding that out online. You would need to have an inside contact with either Medicare or an insurance company. This is most likely confidential information. And even then I doubt if there is a "typical" amount. It is likely to vary.
Sorry I can't be of more help, maybe someone from within the industry can help more.