I’d expect most of the cost of payroll processing is for determining how much to pay each person based on hours worked, salary, deductions, etc. This would all still have to be calculated in order to give someone some odd debit card payout and annoy your employees.
I run a small business. Figuring out what to pay people is part of the cost. You also have to pay the processing company to process. You can do it yourself but then you have more labor involved with filing reports and returns.