Cheapest way is this: (Score 1)

Step One: Take a business laptop or two with a wireless card (preferably with Linux installed, but we can understand...)

Step Two: Walk around the city streets carrying it. Ignore people who look at you strangely.

Step Three: Every block, take a look at your wireless receiver. Write down your signal strength. If you are in an alleyway, pay the local hobos or pay cash to the local drug dealer (note that both add cost to the mission)

Wait, the wireless network needs to be setup? Never mind then...