In your Products List (Settings - Products & Financial - Products List), at the bottom of the products list, turn the "Manage Product Cost Price" toggle on.

Once you have done this, within each product, you will then see a new field to include the product cost price. When you input the cost price, the system will automatically calculate the margin!

Within a booking, the system will calculate the overall Profit/Margin per booking based on all the products/services that have been added.