How many googleads would pay for SF's free Wi-Fi?

Russell Shaw at ZD's IP Telephony blog riffs on the idea that it would cost Google $250 million to deploy Wi-Fi throughout San Francisco. Glenn Fleischman's source offers several reason. Russell take a deeper look at each. Basically, topography is obvious; there are a lot of Wi-Fi networks in place in SF, possibly causing interference; cost of obtaining rights of way is probably overstated - the city controls many high points; electrical costs to run thousands of access points is a real concern. So, $250 million. That number seems  high - by an order of magnitude. Even so, Russell asks, how many ads would Google have to place to make up for this cost?