Setting a global margin for all my products

I run a multivendor marketplace and all the products that are listed by my sellers have a set commission rate of 15% on those products. It is the same percent for every product and I was wondering if there was a way to set a margin on all my products so that the cost of each product would display 85% of the product total price. This way I can view my net sales as well as revenue overall.

I know you can enter in the cost for each product but if you could set a global cost of 85% of the total product price for every product that would be great.