This might be a regional thing, but I have never seen iTunes gift cards discounted where I'm from (Germany). What I've seen is that if you buy a gift card at face value, they give you an extra gift card for 10-15% of the value. And I've not once seen such a promotion for any non-iTunes gift card.
This approach leads me to assume that that either happens in co-operation with Apple, who are eating the extra cost for the special promotion, or the stores are paying out of their marketing budget for a loss leader.
I don't have any experience with the gift card markets in any other country, so things might just be different here.
Either way this is all hypotheticals since I still can't find any good hard numbers.
EDIT: Of course you also need to take into account that not all sales in the stores are paid for though gift cards.
This approach leads me to assume that that either happens in co-operation with Apple, who are eating the extra cost for the special promotion, or the stores are paying out of their marketing budget for a loss leader.
I don't have any experience with the gift card markets in any other country, so things might just be different here. Either way this is all hypotheticals since I still can't find any good hard numbers.
EDIT: Of course you also need to take into account that not all sales in the stores are paid for though gift cards.