hmmmm not sure if that would be a good move at all.
Its bandwidth used (ie amount of data transferred) not bandwidth speed that is where most of the money goes.
Basic connection costs ie exchange, port, backhaul are exactly the same regardless of what speed can be achieved.
The SP's have to sink huge amount of funds into the backhaul costs and ensure that there is enough backhaul bandwidth to cope with demand and along with MSANs, gateways and other routing costs, this is where the vast majority of the money goes.
"Final mile" costs are actually more expensive for the ISP/BToR as these lines tend to be the most troublesome and therefore cost far more in human resources than the shorter lines that can get higher speeds.
You could have user_a on a 24Mbps line downloading 2GB per month and user_b with a 60db line @ say 2Mbps downloading 100 GB per month.
Without doubt user_a is going to be costing the ISP very little compared to user_b not to mention that user_b is going to be far more likely to be wanting and needing more CS support and interaction with BTw/BToR.
Therefore why should user_a have to pay more than user_b when they are actually costing the ISP far less in real terms.
I also have some fears about what this could do to the pricing structure as a whole.. will it actually be a backward step? Whilst I appreciate some general costs have decreased, those of us who had broadband on fixed speeds many many years ago will know just how expensive fixed speed adsl was back then. Bringing in 'up to 8Mbps' opened up speeds for many and better pricing, but as already said above, people needed to understand what 'up to' means and why it is 'up to'.
Its always been my ethos that although I do sympathise some people not being able to get higher speeds, people need to understand that its not any cheaper to provision a 2Mbps line than a 24Mbps line. If this comes to fruition then it may actually be a backwards step for adsl.
The worst part of 'up to' has already been dealt with by OFCOM as there were some ISPs who in the early days would advertise 8Mbps without the word upto being there.. but thats all been sorted now.