According to a report from the Urban Institute, median monthly benchmark premiums directly correlate with the number of insurers participating in a given rating region. The more the number of insurers, the lower are the amount of the premium and rate of premium increase.
The report can be obtained at http://www.urban.org/sites/default/files/publication/90211/2001265-what_characterizes_the_marketplaces_with_one_or_two_insurers_0.pdf.