Each drug plan sets its own prices and “tiers” for individual drugs. That’s the reason for doing the comparison of Part D plans on Medicare.gov — inserting your own prescriptions and dosages. They are all competing to get customers, because they make money on volume. If one company says it costs “nothing” in monthly premiums, you can be sure they are making it up on the cost of drugs. That’s why you really want to compare the “total cost of drugs” — assuming you can get them from a convenient pharmacy that is part of the plan (or agree to do it through mail order).