I think changing/transmuting the looks of Back items (like wings, quaggan backpacks) should cost nothing to let people change them at will – anet would also make more money out of this.
Unlike armor sets, back pieces can go with anything, and people would like to wear them depending on the situation. That means a lot of people who would like to purchase say a quaggan backpack from the store, will not do so because they rather have some other permanent look.
If it was free to change to its graphics anytime you wanted, those items would get more use, feel more rewarding to get them and people would also be more prone to purchase them from the diamonds cosmetic store.
Please consider anet