To inflate appearance of the cost you divide all your non- database marketing $ spent by the total number of new clinets gained during that period. Ad agencies like to use that to get you to use them and tell you they will lower that number.
In reality ongoing advertising will get new customers as well as trigger repeat and referral customers to act when the message hits them at a suseptable time. I don't lend much credence in trying analyze the cost of a new client, but rather whether the overall advertising strategy and execution is accomplishing the goal of getting the phones to ring (remember getting the sale is your or your call center's job) or rugs brought to us for cleaning.