ROI focused bid optimization is the key to keeping your conversion rates high and CPA as low as possible. Many advertisers get distracted by ad position and keep increasing their bids to stay above their competitors. That's not always the optimal strategy, when it comes to ROI. This recipe focuses on identifying which keywords to adjust bids for and how to adjust them based on conversions.
In order to optimize for ROI, you will need to implement conversion tracking and your keywords will need to have accumulated some data that you can work with.
Also, keep in mind your overall CPA goal. That goal will help you identify ad groups and keywords outside and within a profitable range.