I don't know where you are located, but if you are in the USA you can't use prepay.
You might look at the "Budget Optimizer" option...it does the following:
The Google Budget Optimizerô campaign management tool automatically adjusts your keyword maximum cost-per-click (CPC) amounts on your behalf. All you need to do is set a target budget, and the Budget Optimizer will actively seek out the most clicks possible within that budget. Please note that the goal of the Budget Optimizer is simply to help you receive the highest number of clicks possible within your budget. The Budget Optimizer will not help you achieve a specific ad position. Also, the Budget Optimizer is only available for campaigns with fewer than 10,000 keywords.
The Budget Optimizer is ideal for advertisers who:
Consider CPC and/or traffic critical metrics. Have set monthly budgets. Want to automate their CPC adjustment process.
It basically changes the amount of your bid to make sure you spend your entire budget within a month--then you start over.
If interested, check out the Adwords help section for more info.