Some great insights into better understanding your ads spend. It is pretty important to understand your acquisition costs, life time value, churn, sources of traffic, conversion funnel, and customer segmentation.

Overview: http://andrewchen.co/how-to-create-a-profitable-freemium-startup-spreadsheet-model-included/, also included a great Excel model.

Visualize relationship between CAC, Churn, LTV, and Cross Sell: http://blog.asmartbear.com/visualizing-the-interactions-between-cac-churn-and-ltv.html