What you're describing is a co-op. Insurance negotiates with preferred pharmacies to reach a discounted price. That's the price we pay for the drug until the deductible is reached. We end up paying monthly premiums plus copay plus deductible before the insurance pays anything.

The money for everyone else's drugs comes from a shared pool of money from premiums. That's the basic idea for insurance. Insurance companies only make a profit by getting lucky, or denying claims.