Cost Amount when "Must Enter Price"?

We operate a hardware store where we sell a lot of small items (e.g. Bolts, screws, etc..) that we use a generic sku for and have the box checked in the options tab to"Must Enter Price". Is there a way that everytime this sku is sold that the cost can be calculated at the time of the sale to be a percentage of the price entered? Otherwise this really throws off the COGS.

Thanks