Adding cost to an inventory item

SUGGESTED

Hi guys my partner when he first started the company he inputed all the inventory items but he did a pretty sloppy and bare minimum job

He didn't put the cost of the item anywhere, but I'm trying to figure out where to put it now because I want to be able to do a report at every x amount of time that shows
Item xxxxx sold at 9$ but cost us 6$ yield us x amount of profit

How do I proceed?