the 300GB limit is download+upload or just for download?
in the scenario in which the 300GB are for download only you can limit the download speed via simple queue so that the 300GB quota will be met at the end of the month assuming a 30 day month like this:
30 days=2592000 seconds
300 GB=2516582400 kb
=> ~970kb/s or ~121.25 KB/s for the download (if you want to be on the safe side you could subtract 5% for overhead, invalid connections etc).
(i have not taken into account that you will not be using the internet connection 24/7).