This is not hard to do since the ISP can jut apply an IP Profile for a specific speed band
It still doesn't equate to cost though.
If I have an FTTC line capable of 80Mb/s then I could pay for 80Mb/s, 40Mb/s or perhaps add on 20Mb/s, 10Mb/s, 2Mb/s.
The actual cost for BT OR providing me the 2Mb/s version is identical to them providing the 80Mb/s. No difference in operating costs, running costs, maintenance costs, etc, etc. The only difference is how much I use the connection and when as that drives how much backhaul that needs to be bought. A heavy user on 2Mb/s could easily cost more to run than a light user on 80Mb/s.
This is why charging by connection speed doesn't work. That is not where the differential costs are - the differential is in the backhaul.