Contained within the regulations to implement health benefit coverage for young adults are cost estimates and estimates of the number of individuals expected to enroll under these provisions (mid-range estimate 1.24 million new children covered). but this new coverage will add to the premiums paid by all individuals covered by a group, especially those who currently have family coverage.
Here is the cost estimate from HHS:
For those enrolling in their parents’ (employer-sponsored insurance) ESI, the expected annual premium cost under the mid-range take-up rate assumption would be $3,380 in 2011, $3,500 in 2012 and $3,690 in 2013. If these costs were distributed among all family ESI plans, family premiums would be expected to rise by 0.7 percent in 2011, 1.0 percent in 2012, and 1.0 percent in 2013 due to these interim final regulations.
For more on these regulations click here

