Again, you are imaging a scenario in which Zavvi are a specialist. They are not. Despite the fact that they sell a lot of steelbooks, they also sell T-shirts, trainers, electrical goods etc.
Were they a specialist in the vein of FA, Kimchi, Nova, HDz, Blufans they would have the time and resources available to enter protracted negotations with the studios over artwork and appropriate extras; but then your £15-99 steelbook would be a £30 steelbook, and so it goes . . . .
Regarding future values, comparison is odious, but no-one in their right mind would pay £250 for FAC AASB (it's not particularly well finished, the artwork is nothing special, extras non-existent, and the film is pretty bad) but they do!