Its quite complicated as its based on components. The amount and cost of components is based on how you provision it. Backhaul bandwidth and core transit is often overlooked and these are what make up a huge proportion of the ISP costs.
In its most simplistic form 'port cost' is about the only singular monthly item. Port cost is what BT charges the ISP to connect the DSLAM or MSAN.
For 21CN if iirc its £5.88 + VAT and £16 + VAT for 80/20.
Backhaul bandwidth starts running into the millions. These costs vary depending upon how many customers the ISP has and how contended they wish to run their network. Also something like a Juniper to terminate the BT backhaul or host link can also rack up a good £0.5 million each. All these costs are before you even start to think about mail servers, staff, premises and other hardware. There's not much profit in DSL these day... hence why so many ISPs inflate their telephony costs.
Many years ago I wrote an article about the cost of UK DSL... it took a long time to complete and to find out all the component costs and to make an accurate representation. Things have changed since then because some of the costs are now slightly cheaper.. but you can read a summary
here. The full report (Capacity Report) which I wrote for elsewhere caused quite a commotion at the time, yet all the figures were accurate and undisputed by anyone in the industry.
The figures are no longer publicly available for backhaul bandwidth for WBMC, but in comparison a 1Gb EP for WBC (which doesnt traverse the BT core) is £57,060 + VAT pa.