Step 1: Calculate and capture profitability (Margin)
To do this out-of-the-box, sales users can add line items to deals they own, either custom line items or those saved to the portal in the product library - ensuring that, whichever method is used, both the Price and Unit cost properties are populated so the Margin is auto-calculated by HubSpot.
Step 2: Stamp profitability (Margin) up to the Deal
The next step will be to create a custom calculated deal property to pull the calculated line item level Margin up to the Deal record.
The benefit of having the Margin at a deal level is that you can then create a custom calculation deal property formatted as a currency to calculate the commission payment for that deal based on the profitability of it, calculation properties are not available at a line item level.
E.g. If the commission on deals with a margin of less than £10,000 is 10% and over £10,000 is 20% the calculation property would be something like:
IF(([DEAL.margin] < 10000), ([DEAL.margin] * 0.1), ([DEAL.margin] * 0.2))
You'll be able to test your formula within the property builder to ensure the formula is calculating as expected. 
Step 3: Report on profitability (Margin)
As this is a deal property, you can then build a report showing total commission by rep by close month, something like this:
You can adapt this process to meet your business needs, adding in things such as:
- a profitability percentage calculation property
- a commission payment status
- a commission leaderboard
