I have no issue with variable pricing on sync rate. The problem is as kitz reply shows, there is now an expectation for some strange reason that isps should be charging based on their costs to openreach and nothing else.
e.g. the cost of backhaul is often not considered, a line syncing at a higher rate does have inherent higher backhaul costs because there needs to be capacity free to service that line "just in case" the end user utilises the connection, and 40mbps of capacity is cheaper than 80mbps of capacity on a percentile based burst billing model which is what the isp's pay for.
I also have no issue of really paying for what you get and the only way costs would be relevant is that they low enough that its a profitable service. But the UK market at least in xDSL is not really run like this, certainly a factor is the ofcom regulation. The original pricing model before ofcom got stuck in had 2mbps ADSL cost circa £80 month for the consumer vs about £40 a month for 0,5mbit ADSL. The model before ofcom intervention had different openreach ports cost differently based on the burst speed, and also BTw backhaul was much more in line with industry standard pricing. However ofcom wanted to push dirt cheap broadband to homes regardless of sync speed, so ADSL MAX came about with its fixed pricing for all, and BTw backhaul shot up in price to compensate BT for the lost revenue on the new access port prices. This horrible imbalanced model of course led to the dark days of UK broadband where isps were having significant capacity problems due to the cheap deployment of high speed ports alongside expensive backhaul to service those connections. This model is what made LLU so attractive, as there was significant gains to be had on backhaul costs when using LLU. Why the likes of ukonline, BE and sky could offer unlimited usage without throttling.