The millions of seniors who buy Medicare supplements may be charged wildly different amounts — even when they're the same age, living in the same city, and getting precisely the same benefits, Weiss Ratings reports. The reason: The commercial insurers that offer the supplements are allowed by state regulators to charge based on their expenses and costs of present patients. One example: A 79-year-old woman in Aventura choosing Plan F — a comprehensive plan covering almost all out-of pocket Medicare costs — could pay anywhere from $3,654 a year to $5,419 for exactly the same protection, according to Gavin Magor, an analyst with Jupiter-based Weiss Ratings, an independent rating agency of financial institutions. About 10 million seniors choose to sign up annually for these supplements, according to America's Health Insurance Plans. That process began Saturday, with the start of Medicare's annual open enrollment.