A good estimation is $5/month/employee. Quantity discounts do apply. Given that you are looking to drive both operational efficiencies as well as trust between you and your employees, we recommend all employees be setup as users so that your company can realize the value of engagement.
We exist to make your business better. Why shouldn't billing be a part of that as well.
Option 1: If your organization is fairly static in nature and would benefit from a consistent monthly bill, we can use a traditional model that emphasizes predictability.
Option 2: Alternatively, if you have a dramatically fluctuating worforce (construction, seasonal, contract, etc.) we can setup a billing model that is based upon how many active users you had the previous month. We break the montly bill down by day, and take a daily sampling of how many active users were setup in the system on that day, then accumulate for the month. In this way, your bill fluctuates more directly with your workforce numbers.