An efficient budget covers the customer demand that your coupon generates, for the duration that you set. For example, if a product is selling an average of 20 units per day without a coupon, and you want to run a $5 off coupon for 10 days for this product, the minimum budget you set should be the number of days multiplied by the number of average daily units. So in this example (10 x 20) = 200 units should be your minimum budget.
To make the most of your coupon run time, consider estimating a potential sales lift with your coupon and factor that into your calculation. In this example, if the product is selling an average of 20 units per day without a coupon and you estimate a 10% sales lift, consider a budget of (10 x 22) = 220 units.
Avoid creating low budgets (fewer than 500 units) for deep discount coupons such as $35 off. Low budgets for high discounts might cause your budget to expire rapidly after only a couple hours. As a result, only a handful of customers will be able to see and interact with your coupon, and you might experience budget overshoot.
For more information, refer to Why is my spend higher than my budget?.