Coupon on costprice!

[AntonioS28]

Hi, i'm using joomla3.8 and vm 3.2.2
i've a product with these prices:
cost price 10euro
baseprice 10 euro <- 10% discount
final price 9euro
so i sold this product at 9euro.

Now the problem: i want to create a coupon with another 10% discount BUT if i apply it, it gives me the product on 9euro - 10% result: 8.10euro

but i need to apply the coupon on the costprice...
how to do this?