From the Ts & Cs at the Apple Store, Re: gift cards.
"Prepaid cards are not redeemable for cash and cannot be returned for a cash refund, exchanged, or used to purchase gift certificates or provide allowances. Unused balances are nontransferable."
Which to me implies that you can't get back your 15 cents if you use your $15 gift card for 15 singles. Of course, Apple don't force you to buy giftcards, and every other gift certificiate scheme I've seen won't give change either. I prefer apple's system, when I buy a song at iTMS, I see a 0.79 bill on by credit card. More if I buy > 1 song at a time (unusual). They don't charge me 5 pounds and then use that until it runs out, which is what MS are effectivly doing.
Zune Marketplace's Absurd Pricing Scheme