To put it simply , it will only cost how much you want to spend

Explained a bit better below

Costs Associated with AdWords

Because you set your advertisement budget and limits based on what you are willing to pay per click, per day, or per month, you remain in control of your account. AdWords does not require you to commit to a year of service. You simply pay for what you want, no more and no less. When you set up your account you can even select how you want to pay for your advertisement services. AdWords accepts Master Card, American Express, JCB, Visa, and debit cards with the MC symbol or Visa logo.

You can even select when you want to pay. If you want to pay after you have accrued a balance then you will need to set up your billing information and provide payment information at the time you sign up for services. After you have signed up you will receive an initial credit limit of $50. Once you reach this limit your billing method will be charged, and after successful processing of your payment your credit limit will be raised incrementally depending on how many times you hit your credit limit during the month. While not every account will be able to increase their credit limit above $500, AdWords does review each case and request for a credit limit increase.

If you want to pay in advance then you don’t have to worry about your credit limit. You simply pre-pay the amount of money that you are willing to spend for a certain time period, a day, a week, or a month. As your ad receives clicks or impressions your account is debited until your account balance is $0. At this point you can either deposit more money into your account, or wait until your next advertising cycle. AdWords helps you keep track of your account balance by emailing you when your account balance is low.