5 GB a day isn't really that much. That is only like 6kB/sec or 48kb/sec or .0048mbit. Raw bandwidth varies in price as it is almost like a commodity these days. But last time I hear it is around $30/mbit with a commitment of 100mbits or so.
It's just a matter of economies of scale. Not to mention hosts are usually in the same building that major bandwidth is located. When you pay for your home connection, you pay for the transit from a bandwidth provider usually based in the city, to your house.
I'm really looking forward to WiMax. This new format is simiar to WiFi except it is faster and will go longer distances. This could be a last-mile technology that would drastically reduce transit costs. In theory, you or a co-op could buy bandwidth and transmit it using WiMax. Maybe in a few years.
Small Edit: That is probably a typo, most hosts offer 5 gb a month, not a day. But even on a daily basis you see how little that actually is.