Most people fill the gaps in Medicare with a medigap plan, but the price can vary enormously by insurer even though every plan with the same letter designation provides the exact same coverage. You have six months after signing up for Part B to choose any medigap plan; after that, insurers can generally charge more or deny coverage because of preexisting conditions. You can get prices for many policies at www.ehealthinsurance.com or from most state insurance departments. Or order a personalized Weiss Ratings Medigap Report ($49 for Kiplinger readers), which lists premiums for all the plans in your area plus strategies for saving money.