I don't think $10 is too much for a collectible case. Most of the time if you can get it on release day here in the US it is usually only a couple dollars more or sometimes the same price as the amaray. After the first week it goes up about 10.
$15 is o.k. if the steelbook is indeed limited and nicely presented!! Could actually be more than that when I think about it....could have gotten the German "Zombeland" regular for 8€ and paid 35€ for the steelbook instead