What's the rationale behind a download quota? The price of bits is totally artificial. The ongoing costs are hardware and maintenance, power, infrastructure. None of this is any cheaper or more expensive based on the bits that pass through.
The reason for download quotas is to spread the load. The system couldn't handle everyone downloading XXX at once, so by giving people YYY per month, you're encouraging people to self-regulate their usage and spread their bandwidth requirements over a monthly billing cycle.
Why not pay for a set amount of bandwidth? Some people are only going to do light browsing and limited downloading. Give them 5mbps and they could do anything quite comfortably, but you're at a quarter of the line speed. Give me a plan that offers 5-8mbps during peak time and the full 24mbps during off peak. That's more than enough for me to play my computer games and check youtube/etc during peak time, and I can download my World of Warcraft patches and other such large, legitimate items of software through bittorrent in the off-peak times when business is offline.
But of course, that would be less money for the ISPs ...