Click Customer > General tab > Click 
Edit Popup Contents next to each drop down box to display window.
The source of lead window allows you to record what lead your customer came from and the amount of expenditure on leads. If you use this for every customer you can run a report on lead expenditure.
1.Click 
Add on the left hand side for a new row
2.Type a label into the Lead Source column e.g. a newspaper title you advertised in.
3.Type a label into the Analysis Group column to group your leads e.g. Local Press
4.In the right-hand window click Add for a new row.
5.Complete the column headings, date, from and to (dates), cost of the advert and comments (free type notes).
As you record the source of leads for new customers you can run a report to show your expenditure between set dates. To run the report go to Reports > Customers/Prospects... > Lead Expenditure.
Click 
Update to save and close the window.