I'afraid I'm am not aware of any such contrib for SME...
Internet Cafes I know charge "by the minute" not "by Megabyte".
(...which makes sense to me since these days "it's equipment rental time" not octets in places I know/frequent.)
...also: if you want to take stuff home they charge for CD or for pages_printed
good business 
If otoh you want to limit file-downloads because they are blocking your connection...
if you need some bandwidth available for things like HTML,
if you want to limit web access to xxkbit per second for files like .zip .rar .pdf...
if at the same time, you don't want a single user to use more than their fair share of sustained bandwidth
...have a look at SQUID "delay-pool" Definitions
plenty of examples for cut & paste on the web, search via google...
In general I can only recommend:
Bandwidth-Limiting-HOWTORegards
Reinhold