The daily budget is the amount you're prepared to spend each day, on average, for each ad campaign in your account. The amount is totally up to you, and you may change it whenever you want. While setting a bid, I prefer to specify the maximum amount I am willing to spend for one click on the ad, 1,000 ad impressions, or one conversion. Depending on the auction, the real expenses may change. Regardless of how much real prices vary, your daily budget limits how much you can spend over the average number of days in a month.