The first is the actual cost of the plan. If the plan participant would qualify for a lower cost plan, for instance if the plan participant is single with no children, then the "additional cost" must be paid by either the plan participant or their ex-spouse. Usually the ex-spouse seeking this coverage will pay the "additional cost" but this must be defined in a court order or agreement.
In addition, the IRS defines excludaible fringe benefit costs to include only costs for spouses and other dependents. Ex-spouse coverage is not excludible and is therefore a taxable benefit. Although often overlooked by employers, many employers have started to treat these ongoing benefits to ex-spouses as taxable income to the employee.
For more information about the taxation of health insurance benefits read the following article: Health Insurance Taxation Issues Post Divorce by Justin Kelsey and Chris Chen.