How much does payroll software cost?

Depends on what level of payroll software you are looking at.  There are some free programs that will do the basic function for a small business.  And there ones that cost thousands of dollars a year and are designed to track payroll and other information for thousands of employees across hundreds of locations.

Often the price will vary by the features you want and how many employees you will be using it for.

Whatever you're willing to pay for it. There are free solutions, and there are bespoke solutions that cost millions.