Employer costs include contractual mandatory benefits, taxes or any other fees
the local government may require from the employer to pay for when hiring an employee in a particular country.
The total employer cost will vary according to the employment country, and precise state, province and of course the employee salary. For example in Canada, the employer cost will cover Canada Pension Plan, Employment Health Tax, Parental Health Benefit and Workers Compensation Insurance.
For details on the quote process which is used to determine employer costs check out this article.