An efficient budget covers the cost of 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 500 JPY off coupon for 10 days for this product, the minimum budget you set should be: (number of days x number of average daily units) x (discount amount + redemption fee). In this example (10 x 20) x (560). 112,000 JPY should be your minimum budget.
Avoid creating low budgets (less than 50,000 JPY) for deep discount coupons such as 50% off or 3,500 JPY off. Low budgets for high discounts will cause your budget to expire rapidly in a couple of hours. As a result, only a handful of customers will be able to see and interact with your coupon.