According to a recent Health Affairs report, only 5.2% of beneficiaries choose the cheapest plan that meets their medication needs.  Using 2009 Part D data, the report found that beneficiaries spend on average $368 more a year than they would spend had they purchased a cheaper plan.  More than 20% of beneficiaries overspend by $500 a year.

See the report here.  Read more about Part D here.