Create a ROMI Model

Need to create a ROMI model. My client is able to make a marketing investment in a jurisdicction. He want's to be sure that the investment that he is suggesting to his superiors has a solid base. The way that I think that he can use is creating a ROMI projection model based on the past sales on the same jurisdiction; 1 previous campaign that takes place on that very same jurisdiction; takes obviously into consideration the churn, etc. I need someone with vast experience in marketing area. My client is a high profile executive and I need to deliver to him something extremly professional. And...needs to be done today.