If you are talking about Medigap (or Medicare Supplement) Insurance, that would vary according to the company and plan you select. Some Medicare Advantage Plans offer 0 premium. Medigap Insurance premiums could be any where from $30 per month to $300 a month or more depending on where you live, your age, the plan, and the company.
Medicare Part B currently has a monthly premium of $105 for most people in 2013 but will be higher for those with higher incomes.
Medicare Part D is a prescription drug plan available through private insurance carriers and has a monthly premium that varies by plan and zip code.