I can't speak to a formula because there are so many different approaches that it is mindboggling. Probably the most effective way to look at this is a measure that is used often called Customer Lifetime Value, which can identify the profitability and overall value of an individual customer over a "lifetime" with the measuring company.
There are many approaches, though the one that I subscribe to is from CRM industry veteran and guru, Mei Lin Fung. To find it, go here and you'll get an idea. For more detail, get CRM at the Speed of Light (my book) and check out Appendix A in the second edition; or email me and I'll send you the information. My email is firstname.lastname@example.org.
All the best,
This was first published in December 2003