If we assume I'm a contractor charging $100 per hour and take the cash as salary:
(100/1.3142) * 0.5 = $38 left after payroll tax and the top income tax rate (50%).
So if I'm just as fast and good as a professional, the professional needs to charge $38 per hour or less for it to be worth it. But in reality, a professional is probably at least 2x as fast as me, making it $76. That's probably less than they charge, so it's worth paying for a professional.