To calculate the margin of your event, you can add the cost price of your products.
Go to setup > products to add the cost price.
Adjust the cost price in the event dasboard in case of any exceptions.
Pay attention! Just like the product price, the cost price is not automatically adjusted if you adjust or add the price via setup.
Click on 'refresh product prices' to retrieve the new price and cost price. To do this, go to the event dashboard and select the 'gear' icon on the top right (as shown in image below).