On the sales order form, I can't find a way to put a "monthly Cost" to have the quantity in order to multiply it by total months in order to get the total yearly cost for the Sales Order. 

For example, i have someone who is buying 10 units per month at $300 per unit at a monthly cost of $3000 for 12 months which would make the Sales Order total $36,000.  Can you help me with customizing this? I've tried going into products, but its no help.

Thank you.

Pat