The relationship between cost$-$per$-$click and cost$-$per$-$acquisition is the most important issue you will face. This is the measurement of whether or not your SEM campaign is making any money for the company. Read the section in the Yahoo guide that starts on page $47$ for a basic explanation.

Find an article online to share with us that, $1)$ is a case study showing how CPA was used and measured, $2)$ has tips for measuring CPA, or $3)$ suggests how to maximize the CPC budget. The article might include all these elements. If you locate one that isn$$t exactly about one of the three items but you feel would benefit the class, use it$.$

Try a general search or sites like www$.$marketingprofs.com, eMarketer, $1$to$1$ media, Advertising Age, About.com, www$.$hubspot.com, etc.