They bank on everyone not using their full quota... and that is in part is what is used to increase the limits to higher than what they would be otherwise.
A few years ago I did an article on the pricing of IPstream adsl and how much it costs the ISP (before they have to add in such things as premises staff and equipment etc).
Some of the article is reproduced
here.
I dont expect you to read it - but it basically sumouts to approx £9 each month port cost at the exchange and £1 per GB.
Prices have changed a little since then - but not much... and the pricing in that article was confirmed as correct by several sources (adsl guide ran a similar one soon after with similar results).
Believe it or not theres not that much profit in adsl as companies compete to lower their prices.. and several organisations (such as CPW or Sky) subsidise it from income from other areas. (Something which BT is not allowed to do).
The only way to make it fair across the board is make sure that everyone pays the exchange port costs to their telco.. and then the ISPs charge everyone for each and every GB used on top of a base charge for their running costs. Kinda like we do with Gas, Electric, and in some cases water etc.
