According to a recent analysis of the projected effect in the 34 states

... in which the federal government rather than the state is operating the health insurance marketplace under the Affordable Care Act (ACA), if the Supreme Court sides with the plaintiff in the case, King v Burrell:

• As of mid-February, 7.5 million people had signed up for coverage for 2015 and qualified for a subsidy in states using the federal marketplace
• 87% of those who have signed up for coverage for 2015 receive subsidies
• The subsidies average $268 per month, per person and cover 72% of the premium
• If the subsidies were eliminated, former recipients would see an average increase of 256% in their out-of-pocket premiums

Source: "Insurance Markets in a Post-King World," The Henry J. Kaiser Family Foundation, February 25, 2015, http://kff.org/health-reform/perspective/insurance-markets-in-a-post-king-world/