One of the most important figures a marketer needs to focus on is "lifetime customer value." In other words, how much money will a client bring to you, on average, during their entire relationship with you?
Read this useful explanation of the concept in the Duct Tape Marketing Blog.
Why is it important? Because if you know how much a client is worth throughout the entire time they are with you, you know how much you can spend to attract that client in the first place.