To cover London in CPUs roughly as powerful as the brains of the Android tablet I'm reading this talk from, to a density of one per square metre, should therefore cost around €150M in 2040, or €20 per citizen. To put this in perspective, in 2007 it was estimated that councils in London spent around £150M, or €190M, per year on removing chewing gum from the city streets. So for the cost of removing chewing gum, a city in 2030 will be able to give every square metre of its streets the processing power of a 2012 tablet computer (or a 2002 workstation, or a 1982 supercomputer).