Another conclusion you could draw is that when you remove the hype Banksy's work just isn't very compelling (anymore, or perhaps just on canvas). Or perhaps this is evidence of the opposite: a fair number of people were willing to pay $60 for a small monochrome, hastily-sprayed stencil piece.
When it comes to street art/graffiti, the context is just as, if not more, important than the art itself. For example, massive roller paintings on the sides of buildings or pieces on trains are interesting partially because of the danger involved in putting up the work in the first place. When the context is removed and it's just put on a piece of canvas, it loses a lot of its significance. It's a similar reason why some feel that paid murals aren't true graff since then it's not really street art, it's just art on the street.
This. part pf street art is the 'vandalism' aspect. in banksy's case there is almost always an ironic play on this. in others, we've seen thinkgs in the uk like using "powerwashers" to do grafitii, as a relief of light (clean) against normal (dirty). oh, and this selective washing was "illegal", and deemed "vandalism" even though by most sane standards washing a wall (however selective) would seem in the public interest. But the marking of the wall (claiming it as your own) is the inherent social transgression...sad statement on our modern world. Yes, and none of this gets communicated with the same techniques ex-situ.
