Buy specific item get 1 lesser value free
I want to do a promo where if you buy item number "X", you get a free item of equal or lesser value free. I have excluded products, so I've added a field to the database that puts a "discountable price" in the cart and all promos work on that figure rather than the subtotal of the cart.
I'm not quite sure where to start. I know my rule will begin with the item present, then, what?