The problem with most marketing is that you don't know your return on investment (ROI). By calculating your customer lifetime value (LTV), you'll be able to start predicting ROI on your marketing spend.
The following calculator will help you find out the average LTV for your customer segments. For each field, just use an average number (if you have many customers who spend $10,000/year, and many others who spend $1,000/year, then just write $5,000 in the yearly revenue).
If your client has multiple customer segments, run each segment through the calculator so you know which one to target. Consider this:
If both segments are easy to convert, I'd suggest we target segment #2 because the return from each customer is so much higher.
Start by entering your data here: