I like the idea, but I think you're over complicating it with the whole billing stuff.
I would setup a system where for $7/mo. you can have up to 3 MAC addresses registered in the system. All the AP's would be configured with a known SSID and wpa2 key and the 3 device Mac addresses are registered in each persons access point. (you might have to run dd-wrt or tomato on the routers).