Typically, the employer has no control of the cost of your COBRA premiums. That is controlled by the insurance company which provides the group coverage. Further, employers are not required to pay your COBRA premium unless you execute a contract, or have a severance agreement, which obligates the employer to pay the COBRA premiums.
If the employer IS contractually obligated to pay your premiums, you should immediately notify the employer, in writing, of the additional cost and demand that they pay the difference.
If your employer was just helping you out, but has no contractual obligation to pay the COBRA premiums, perhaps you can talk to the employer about reimbursing you for the difference.
If you are unsure if the employer is contractually obligated to pay your premiums, you should consult an ERISA attorney in your area.
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