The cost of Obamacare could rise for millions of Americans next year, with one insurer proposing a 50 percent hike in premiums, fueling the controversy about just how "affordable" the Affordable Care Act really is.

The eye-popping 50 percent hike by New Mexico insurer Blue Cross Blue Shield is an outlier, and state officials may not allow it to go through. But health insurance experts are predicting that premiums will rise more significantly in 2016 than in the first two years of Obamacare exchange coverage. In 2015, for example, premiums increased by an average of 5.4 percent, according to PwC's Health Research Institute.