Draytek and Billion routers both provide access restriction by time for specified device.
Not convinced they do limits of data transferred though - the Billion 7800 certainly doesnt. The Daytek 2820 MAY do - it counts total data transferred from the WAN (which I havent found on the Billion) but I am not sure as to data counts by device. It MAY do - worth a check of the manual on the web site.
Ex <n>ildram , been to SKY MAX - 15,225 Download
BE Unlimited - 21,000 Download 1,200 Upload ON THE LINE THAT SKY COULD ONLY PROVIDE 15,255 DOWN AND 800 UP ON!!!,
Moved house, now BE Unlimited 6,500 Down, 1Mb/s up - gutted!
FTTC Cab installation commenced 12th April - expect full 80 / 20 - bye bye BE, hello BT Infinity, install date Friday 10th August!