You may want to search for "PPACA" and "ACA" on this Forum for earlier discussions of this topic. http://www.ncsl.org/documents/health/defincacamedicdprov.pdf
addresses the definition of income.
You should check on the national site for the health insurance marketplace & then link to the site for your own state. There should be a link to a calculator (I believe sponsored by the Kaiser Family Foundation) that calculates your subsidy level based on Modified Adjusted Gross Income.
Because not all providers support Medicaid, some have decided that they want income to be high enough to not qualify for Medicaid, but low enough to qualify for a subsidy.