For every campaign that you run that aims to make money (which would exclude, for example, brand awareness campaigns), you should be looking at your Return On Investment (ROI).
ROI is a simple calculation that works out how much you've made in comparison to what you've spent.
For example, if you spent £2,000 on a marketing campaign and made £3,000, your ROI would be 50%.
It's calculated by taking your expense from your income (which equals your profit), dividing by your expense, and times by 100, as it's usually expressed as a percentage.
(Income - Expense)