If 400 customers share a 10 Gbps uplink to the backbone, they each need to pay about 0.25% of the cost*. On the other hand, if he were hosting xvides.com over that connection, he'd be using it 10% of the time (averaging 1 Gbps). The 10 Gbps could only handle about 8 such customers, so each would need to pay 12.5% of the cost.
Or use quotas. Quotas are actually okay if they're clearly advertised don't try to trick yon into overage fees. Just set a reasonable quota and just throttle you down to say 10 Mbps afterwards with the option to purchase more at a reasonable cost/TB. I used to do that at my cabin with my cell phone, one or maybe two months a year I'd pay extra and I found that totally fair. Didn't need it all year, never expected unlimited and in proportion to their other subscriptions I got a fair price that was somewhat more expensive those months but cheaper than having it all year.