Acho que um termo que seria mais / mais preciso seria "cota de transferência".
Cota :
a limited or fixed number or amount of people or things, in particular.
O artigo "Cap" da largura de banda da Wikipedia menciona também (especificamente "Cota de download"):
A download quota is a restriction fixed by some ISPs (Internet Service Provider) to limit the volume of data downloaded by the end user during a fixed period, usually a month. Once a fixed download cap (e.g. 250 gigabytes/month) has been reached by the end user, the speed at which they access the internet is usually throttled to a slower speed, cut off, or they are charged for excess data usage.