The cost of Medicare Part B is standard for most people but about five percent of beneficiaries could pay more due to higher incomes. Premiums for Part B are based on an individual’s modified adjusted household gross income. Modified adjusted household gross income is calculated by adding together any money earned through dividends from investments, wages, capital gains, wages, Social Security benefits, and pensions The Social Security office will use your tax returns from prior years to determine how much you will have to pay monthly.