what about making something similar, but where the cost formula is an^2? a should be some constant factor like a tenth of a cent and n should be the number of c

what about making something similar, but where the cost formula is an^2? a should be some constant factor like a tenth of a cent and n should be the number of chars, i.e. per message or in the last 24 hrs. If Twitter worked that way instead of an arbitrary 140/280 char limit, it would be so much cleaner.