Add custom markup% to Expense lines to be Invoiced

I would like to add one column by expense line item, "Markup%." So, when I make an invoice and I have an item that is an expense recorded as $100, I can enter my markup%, i.e. 20, into the “Markup%” column. Then, it does the calculation and enters $120 into the client invoice “Unit Cost” column. Maybe an additional column is needed to indicate that the expense has already been marked up.