Controlling Internet Download Volume on Home Network

I am looking fo a solution for my home network.

I have a wireless network with 3 PC's and an XBOX360 attached.

My ISP provides me with a fast 10Mbit cable network that has a 10Gb monthly
data volume limit, after which I am throttled back to 64Kb/sec for the rest
of the month.

2 of the networked PC's belong to my teenage children and monthly data usage
has gone through the roof since they started using MySpace and YouTube (go
figure). Mainly becuase of the access to video content from firends etc.

Where as previously we had an average monthly use of 3-4Gb, we're now
getting up over 8-9Gb each month.

Rather than increase my plan - for which I'd pay a chunk more money for only
a small increase in data per month and would have to sign up to a 24 month
plan, I'd like to find a way to allocate a data volume/limit/allowance to
each of my children on a monthly/weekly basis.

I have a DLink entwork that is firewallled and Windows Vista on (Home
Premium) on each PC and installed Norton 360 on all of them and am happy that
unwanted content is blocked sufficiently.

I know I could use Windows Vista Account Controls to limit the times my kids
get access to the internet and to limit the sites they visit or to limit
downloads all together and I can do the same using the software on my
Wireless Router but what I really want to do is allocate a data
limit/allowance so that my children (and I) can download what we want
(subject to the security/parental conent controls we've already set). That
way, my children begin learn the value of what they have access to and can
take some control over what they download based on it's imprtance to them.
I'd rather they had the choice to use or save their "allocation" base don
their own needs.

It would be great to be able to set that weekly so that if they "blow the
allowance" they don't have to wait a monmth to get it back and I can monitor
it weekly/monthly and adjust our overall needs accordingly.

I am happy to consider purchasing a utility/application to help manage this
if that's the only way to do it but it would be great if Windows Account
Control or a Windows Network utility could provide that level of control.

Any ideas/suggestions?