assign automatically an expense to an item in zoho invoices

assign automatically an expense to an item in zoho invoices

hi


I need to assign automatically an expense to an item in zoho invoices.

example:

Item "A" price is 100,
item "A"  expenses are 40. 
10-40=Gain is 60. 
This should go automatically per every item without bill to the client of course whit different prices and different expenses for every item. If I need, it must be possible for me to change the amount of the expense.
I want to avoid to record every time a fixed amount of material that is always the same and then to assign to a specific invoice.

Is there any possybility to do it?

thank you in advance for your reply