Time Warner just kicked off their trial on pay-per-use billing.

The limit is set at 5GB per month, anything above that and the users will have to pay extra on top of their bill!

They plan to offer tiers with monthly limits of 5GB, 10GB, 20GB or 40GB

Judging by other ISPs TimeWarner will probably want something like $1-2 per GB above the limits. Even $0.50 will be a lot higher than what they probably buy the traffic for considering it's on their network.
Hopefully not though

http://www.multichannel.com/article/CA6523660.html