>An employer is willing to pay an extra 20%...

I'm sorry, but this is an accounting fiction. The cost of an employee to an employer is the total cost; dividing up the total cost and ascribing it to this or that is irrelevant from the perspective of the employer, and disingenuous from the perspective of the employee.

In the US we have social insurance taxes which are sold as having an "employee" portion and an "employer" portion, but this too is just accounting fiction:

    Perceived:
     Gross Salary:      $100,000
     Employee Tax Rate:     7.65%
     Employee Tax Amt:    $7,650
     Net Salary:         $92,350
     Employer Tax Rate:     7.65%
     Employer Tax Amt:    $7,650

    Actual:
     Gross Salary:      $107,650
     Net Salary:         $92,350
     Tax Amount:         $15,300
     Tax Rate:              14.2%

The employee is worth $107,650 to the employer; that part of the money goes to taxes, or to a recruiter, or to a gym membership is wholly irrelevant.