I found this helpful rule of thumb to estimate the cost of benefits:

https://beebole.com/blog/how-to-calculate-the-real-cost-of-a...

>...the real salary of an employee ends up “being 18% to 26% more than a worker’s base salary.

Also, for an on-site employee, the company would need to add the cost of office space, equipment, IT support services, etc.