Colorado state continuation is in some ways less generous than the Federal plan, but there's not a whole lot of difference between the two.
Federal COBRA law states that employers who have more than 20 employees must provide the option of continuing coverage to all employees who terminate employment for any reason except gross misconduct. The former employee pays the same cost as the employer is being charged by the insurance carrier, plus 2% for administration. (While technically the employer can opt out of the 2% and charge the employee exactly what the insurance carrier charges them, few if any do, nor do I see why they should. COBRA administration is no picnic. I know - I do it.) It's not a question of negotiating a mutually agreeable rate: this is what the law states.
If they made an honest error as to the rates, they can correct it. In fact, if the insurance carrier begins charging them a different rate, they can legally pass that change on to you. To fail to do so might ultimately lead to discrimination charges, if some employees are being charged the full rate and others are not.
Since severance is not legally required, I see no reason why they should not be able to correct an honest error in this way.