SF Google Wi-Fi pegged at $250 mill; now how many ads would it take to cover the nut?

According to at least one report, if Google wants to build a Wi-Fi network that will blanket most all of San Francisco, they will have to spend in the neighborhood of $250 million to do so.

Glenn Fleishman of Wi-Fi Net News says that a telecom analyst in SF tells him the cost will be driven by several factors. Let's look at each:

Topography- There's a reason for the line "little cable cars that reach halfway to the stars" in that song "I Left My Heart in San Francisco." This is one hilly place, and hills aren't all that kind to reception.

Density of Existing Wi-Fi Installations- I just performed a search on Ji-Wire for existing hot spots in SF. The search engine found 438. My hunch says the number is somewhat higher. Given the clustering of these facilities in certain neighborhoods, the potential for interference is quite high.

Cost of Obtaining Rights Of Way- We are talking a very compact city with lots of landowners, building owners.

Basics of Electricity To Handle Thousands of Access Points- Obviously this is an ongoing expense. How much? Here's Pacific Gas & Electric's latest commercial rate sked (Excel format).

OK, now Google says this service would be ad-supported. I guess they mean Google AdWords and AdSense ads.

My question is: if Google plans to make this a going proposition rather than either a loss-leader experiment and/or a demonstration in techno-philanthropy, how many of these ads would Google have to sell to cover that $250 million nut?