This is one of these issues where the rules are being made up as they go. the tax credit was supposed to be based on cost, now it’s based on average cost for the state. See below:
If the employer’s plan costs more than the average plan, the employer can deduct only the amount the employer would have paid for average-cost coverage, officials write.
The table the IRS has published in Revenue Ruling 2010-13 gives what the IRS believes to be the average cost of individual and family coverage in each state.