OPM is not going to do case management that is done by the health care provider.
OPM is going to examine and negotiate the total plans that go into the state exchanges. They are then going to have web sites where you go to find the plans in your area.
The additional cost?
Maybe a couple of million.
OPM is already doing this same function for federal employees, already has the skills and the software.
To see how it will work you can go here and pretend you are a federal employee. Put in your zip code and see the plans for federal employees in your area.
http://opm.gov/insure/health/search/plansearch.aspxAs for the savings the MLR for the health industry is about 35%. The MLR for Humanas which specializes in government plans is 18%.
Beyond that you have additional savings from the cost savings of scale and, even more importantly, you will be getting people to see a doctor early and deal with prevention instead of expensive corrective treatment.
For example free prenatal care will cost pennies on the dollars but it will have a dramatic impact on reducing premature babies and other problems that will save hundreds of billions. We have already seen this work by having immunizations.