In broad terms there is a fixed cost per customer, then add the variable cost of the amount of data they consume. You then need to add a profit margin.
Those on the lowest tariff, as I was, would be only marginally profitable if they approached the 200GB allowance. Maybe even loss-making. As I rarely exceeded 60GB per month they no doubt made a decent but not huge profit from me.
Over time the wholesale costs of usage are reducing. Similarly over time the fixed costs rise, such as building rents and rates and staff pay for every member of the company. The skills of AAISP support answering the phone cost a lot more than those of BT call centre staff.
The higher up the usage scale you go the more effect cost reductions have and the smaller the proportion of the (rising) fixed costs. (That fixed costs can rise is not a contradiction - to an economist fixed costs are the unavoidable ones to run the company for a given number of customers).
The result of that, as fixed costs per customer rise and variable costs fall, is that there is more scope to reduce the price to high users than there is to reduce it for low users.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - Three 4G, tbb tests normally 35-45Mpbs down, 65Mbps off-peak, 9-24 up.
If you never think of anything off the wall, you'll never think of anything original.