But when you pay someone to put up a billboard, there isn't a risk that they'll put it by a road they think gets tons of traffic, but is actually just used by one guy being paid by the building owner to drive past it repeatedly.
...and you also don't get thousands/millions of billboards put up for $500. This part of the comparison should probably be ignored as it is more to do with the intrinsic differences between the physical world and the digital "world" than it is with banner ads vs. billboards.
