In the long run all I see high per GB overage models doing is pushing customers to lower tiers, or at lease it does for me.
For our house it's simple, we have two Xboxs that we use to stream Netflix mostly in HD, we have no cable television. On average between Netflix, the Internet, and everything else we do we consume about 9GB of data a day. I pay about $45 a month for a 7mbps Time Warner cable connection which is about the most I am willing to pay.
If my internet connection cost at my current consumption level goes over $60 I will go to the basic $19.99 768kbps connection which is enough to surf the Internet, play Xbox live, and I will get DVDs in the mail.