GradeMyAds is a free web app which provides you with an instant summary of your Facebook ads performance, including your Average Cost per Engagement, the performance of your latest ads, the total spend, the number of people reached, and the top scores for your best performing ads.
Here is a sample report: http://www.grademyads.com/sample.png
100% Free.
My comment: Instant checkup for your FB ad spending from an external perspective. Good summary report helps you see the relevant data and evaluate where you have done best.
Try it out now: http://www.grademyads.com/