According to this article from Politico, the cost of operating the health insurance exchanges, including the costs of providing subsidies to lower-income purchasers of coverage, will increase dramatically over prior estimates.  In part, this is because of the failure of some states to embrace the expansion of Medicaid.  The cost of Medicaid coverage is $3000 less than the cost of providing subsidies for private insurance on the exchanges, according to current estimates. If insurers then raise premium rates beyond current projections, the difference will grow, and the cost of running exchanges will accelerate. By 2021, the subsidies are now estimated to consume $606 Billion.