Short answer: COBRA generally costs the full premium (the employee share plus the employer share) plus up to a 2% administrative fee, which is why it often feels much more expensive than payroll deductions.
COBRA continuation coverage is usually more expensive than what you paid while employed because your paycheck typically reflected only the employee portion of the premium. Under COBRA, the plan can require you to pay the entire cost of coverage.
In most cases, the amount you can be charged for COBRA coverage cannot exceed 102% of the cost to the plan for similarly situated individuals who have not had a qualifying event. That 102% total is the full premium (employee and employer portions combined) plus up to a 2% administrative fee.
For example, if the total monthly premium for coverage is $600 and the employer previously contributed $400 while the employee paid $200, the COBRA premium could be up to $612 per month ($600 plus 2%).
Even though the price changes, the coverage itself generally does not. COBRA is a continuation of the same group health plan benefits, networks, and rules; the main difference is who pays.