There are four types of costs associated with Medicare Part D prescription drug coverage: premiums, deductibles, copayments, and a coverage gap during which period you must pay the full cost of your medications. People with low incomes may apply for a subsidy from the Social Security Administration to reduce these costs.
In 2013, Part D premiums range from $0-$50 per month (depending on the plans available in your town and on the partiular plan you choose). The deductible -- the amount you must pay out-of-pocket before Medicare will contribute to your prescription costs -- for most plans in 2013 is $325. After you meet the deductible, Medicare will pay roughly 75% of your prescription costs.
After you and your plan together pay a certain amount for covered prescription drugs ($2,970 in 2013), your plan stops paying and you must pay the full cost of the prescription. The plan begins to pay again -- and pays 95% of all further costs -- when total expenditures reach a "catastrophic" level ($4,750 in 2013).
However, you will receive a discount on the cost of your medications while you are in the coverage gap.
For more information, see our article on Medicare Part D basics.