How much does it cost for extra employyes to go onto the payroll?

Welcome to the Community, @rwalsh. The cost per extra employee once QuickBooks inclusion exceeded is $4 per month for Standard Payroll and $6 per month for Advanced Payroll. 

For your reference, check out this link: https://quickbooks.intuit.com/au/payroll/. 

Here's an article for your future reference: How to set up an Employee in QuickBooks Payroll. It helps you learn about adding employees in your payroll account. 

Let me know if you have additional questions, @rwalsh. I'm always around here to help.