Very true, although there will be a significant reduction if cells are bought in bulk. You can buy aftermarket powertool batteries of
~100wh for just over 30GBP, including housing, BMS etc. They wont be the same quality of course, but it does give a comparison.
For a second Ebike battery, if...
It must be only a matter of time before someone makes an aftermarket battery. there must be money in it with the price the OEM ones are.
It also wouldnt suprise me if reconditioned batteries with new cells in start to appear soon. I hope so anyway!
Does the motor store a log of changes made to it? I know the risks to warrenty by making any changes, but they are obviously increased if there is a record / log which can be read back by the dealer.