How much do you spend on digital ads each month? If you don't have a budget for this yet, start by thinking about 10% of your total marketing budget as a good place to start. Once you've entered all other metrics, come back to budget to see how it affects profit.
How much would you be willing to pay for a click?
How many visitors to your website convert into leads?
What percentage of your leads turn into customers? This is a good way to monitor the effectiveness of your marketing campaigns.