HIV-medication and Insurance, how do they work?

I am new to this whole insurance and HIV business, I have insurance Blue Cross Blue Shield of Alabama

And i want to know say if HIV medication cost 1500$ a month, how much do the insurance companies pay and how much do i have to pay? Its a scenario so no spcific answer is required, can they deny you of medical expenses such as HIV-medication altogether because it is so HIGH? How do they decide?