Premiums for some medical plans to be sold to the uninsured next year will be 18 percent less costly than government analysts expected under President Barack Obama's signature health-care law, his administration reported. A benchmark measure of the Affordable Care Act's promise shows monthly rates for "silver" plans will average $321 in 11 states examined in a report today from the Health and Human Services Department. Congressional Budget Office estimates from last year saw silver premium rates averaging $392, HHS said. Obama touted the findings at a White House event designed to promote the benefits of a law that his Republican opponents in Congress are trying to derail before the core provisions take hold Jan. 1.