Hello
most ISP's are still charging 50% plus for 1Mbs. Probably 1Mbs should cost no more then 20% above the 512kbs price.
Don't forget with a 1mb ADSL connection this is twice as fast as a 512k connection (obviously, you might say) but this also means you can potentially use twice the amount of bandwidth in a month - not jsut download faster but download a lot more too.
A 512k connection running at full pelt for a month can download about 160GB of data; a 1mb connection running equally to the max can shift about 310GB of data. What you would do with all of this is anyones guess, but anyway...
Bandwidth costs ISP's money, a lot of money (ie the fees from the tier-1 networks to carry the traffic between one ISP and another); there are a lot of hidden costs to this - such as network routers, switches, fibre cables etc etc, (CISCO kit is not cheap), on top of this the more users on 1mb there are, the bigger the connection pipes between the ISP's and BT need to be, again to cope with all the extra bandwidth. This all has to be paid for somehow.
Therefore as more people use 1mb connection the costs to ISP's would also potentially double too as the amount of data the ISP's networks need to carry would also potentially double, all these costs need to be paid for somehow, so I think its unlikely you will get near a 20% premium for a 1mb connection over 512k too soon.
Kind regards
Doug