You can configure a cart price rule with any Free Gift action to offer your customers a discount on the promo item. To do so, you need to navigate to Marketing > Cart Prce Rules >[select your rule] > Promo Items With Discount and add your discount amount to this field:

It is possible to add fixed price, fixed discount and percent discount to your promo items. 

As a result, the promo item will be displayed with a discount: