How do I budget for commission based independent contractors?

There are a couple of ways to manage independent contractors - in the Employee Module, or outside it.

From within the Employee Module, you would add your contractors as normal employees, but you would probably use a different GL Account as your target category, and not apply any of your standard HR Formulas.

Outside the Employee Module you can simply add the Contractors as sub category items within the appropriate GL Account.

If these contractors are paid solely by commission, then in the Employee Module you would set their salary to zero, and create a commission formula driven by whatever metric you are using to derive that commission (click here for more on commissions).

Outside the employee module, you would create C2C formulas to calculate their commissions (revenue driver x commission rate).