nigelivey :
QOS will not do this it will only prioritise traffic not cap bandwidth which is a whole different ball game. consumer grade equipment rarely has this feature set.
Nigelivery is correct, QOS can permit you to set cell phone on a lower priority to your gaming pc for example, but it does not cap bandwidth per device. mostly used for streaming reasons, QOS prioritizes videos over regular network things to keep film streaming smoothly on big HD TV
I have heard of Gargoyle, a flash firmware replacement for a limited number of home routers that would (I never tested it and please understand I know nothing about this program aside what I have read from web page) do what your looking for, limiting speeds/quota of amount of data...