The best way to keep that client is to find a subcontractor who can do the job. In that way, you move from being "a programmer" to "someone who solves computer problems." When you have a client who is willing to spend money on you, take the money and find someone who can do the work they need.
Make sure that you price the ...
To answer the question, you have to consider beyond the headline cost...
With an employee, as well as the salary, you have to factor in (depending on the jurisdiction):
Additional payroll taxes
Other health care and/or social security charges
Sick pay and holiday pay
Training and development
Provision of equipment
With a ...