Network traffic prioritization by specific cost

I've been following with some interest the recent Comcast scandal where the ISP started inserting TCP reset packets into BitTorrent traffic of its clients, thus disrupting it.

Comcast claims they do it to protect the ability of "normal" users to use the bandwidth from "excessively using" users that hog it.

(I personally believe Comcast tries to minimize the overall amount of traffic that flows out of its network to other ISPs as it costs them money in the fiscal clearing of cross-ISP traffic.)

But let's entertain the idea for a moment that Comcast would really like to protect their "normal" (read: low-traffic) users from their "excessively using" users. How would I go about implementing a fair system that achieves this goal?

Well, certainly not by disrupting the traffic.

Below is an idea that I believe would result in fair allocation of bandwidth within the network of a single ISP with paying customers. I must forewarn you that while I design software systems for living and often need to deal with management of limited resources, I'm just a layman when it comes to the narrow field of packet-switched network traffic management, so what's written below might not make sense at all because of some arcane aspect or the other that I know nothing of :-)

Anyway, here it goes:

The ISPs already measure the amount of traffic generated by every user within a billing period, that's how they are able to detect "excessively using" users today. However, instead of handicapping a user that "excessively" uses the bandwidth, the metrics would be used only to select which user's packet to drop when there is insufficient bandwidth on the routers' outbound lines, but only then.

The strategy would be to always drop the packet coming from the IP address of the user who had the highest traffic in the billing period so far. Or, in case of differently priced packages, or even different billing periods, the user with highest traffic/fee ratio for his current billing period, expressed nominally in byte/dollar.

This way, the proverbial "only checking his mail" user would always experience a fast connection, followed by a user who listens to online radios and watches some amount of online videos, while the "excessive" p2p users would be only competing for leftover bandwidth among themselves.

It'd be easy to suitably extend the idea to a nondeterministic scheme where a packet to be dropped is picked randomly, but the probabilities are weighted by their user's dollar/byte ratio. That'd give a user who heavily used p2p at the start of the month a standing chance to still be able to check his e-mail even when another user comes along who floods the network with p2p traffic near the end of the month.

Feel free to point out why this wouldn't work, doesn't make sense, or is already invented in this or similar form and used somewhere :-) I've tried uncovering this technique using Google, but have found only port/service/application based QoS prioritizing, and didn't find anything specifically describing this -- namely, prioritizing based on user's specific monetary cost of traffic, amortized over a billing period.

As far as I can tell, this method preserves network neutrality, as it does not discriminate based on either the type of traffic or the destination of the data. It also doesn't degrade the throughput for anyone in any way as long as there is sufficient free bandwidth.