A $0 premium plan can still be the expensive plan. You pay a premium every month whether you fill a drug or not. You pay a deductible, copays, and coinsurance when you do. The honest comparison is premium plus expected drug spend at your pharmacy, for your list, for the year.
Part D has phases. After the deductible, you pay the plan’s copays until you reach the annual out-of-pocket cap. Extra Help collapses a lot of this into small set copays and often a $0 deductible. That is why two people in the same ZIP, on the same plan, can have wildly different years: one has Extra Help, one does not.
ThriftMeds estimates are for planning. They are not an official Medicare.gov quote and they are not a promise from a carrier. They exist so you can sort plans by the number that will hit your checking account, not the number in the television ad.
If your medication list is incomplete, the ranking is theater. Add the expensive drugs first—insulin, inhalers, anticoagulants, oncology—then the cheap generics. Re-run the comparison if a clinician changes a dose in November. Annual Enrollment is when the list should be current.