The average price tag for the most popular level of insurance sold in the Affordable Care Act's federal marketplaces is dropping slightly, the first time the rates have stopped going up since the health plans were created a half-dozen years ago. In the 39 states that rely on HealthCare.gov, the monthly premium is dipping by 1.5 percent for 2019 in a tier of coverage that forms the basis for the ACA's federal insurance subsidies, according to federal figures released Tuesday.