How much you pay per month depends on your adjusted gross income. By Kimberly Lankford, Contributing Editor July 29, 2008 Editor's note: This article is adapted from Kiplinger's Retirement Planning 2008 guide. Order your copy today.Last year, Medicare instituted a two-tier system for Part B premiums, with upper-income retirees paying more than others. Premiums and income limits rose moderately in 2008. The premiums are based on 2006 income because those are the last tax returns the IRS has on record. Now, joint filers with an adjusted gross income of $164,000 or less in 2006, and single filers with an AGI of $82,200 or less, pay the base rate of $96.40 each per month. People with higher incomes pay more. The premium is deducted from your Social Security benefit. Joint filers earning up to $204,000, and single filers earning up to $102,000, pay $122.20 each per month. Premiums gradually increase based on income, up to $238.40 per month for joint filers who earned more than $410,000 and single filers who earned more than $205,000. Advertisement But you may be able to get your premiums reduced if your income has dropped since 2006, under certain circumstances -- and retirement is one of those reasons. Other eligible life-changing events include the death of a spouse, marriage, divorce or a reduction in income. If you've experienced any of those changes, contact the Social Security Administration at 800-772-1213 or visit your local field office.