In the zone
Try to configure Bridge Connection in the modem and use dial-up for connecting internet. Write 2 small batch files (google it) to connect & disconnect the modem. When the connection gets disconnected, every downloads stop and also the remaining time will not get charged (I am sure about that) even if the modem is on. Use Windows task scheduler to schedule running of the batch files.
I am using 500C and scheduled to run the connection .bat file at 2:15 AM and disconnection file at 7:45 AM and I use a Auto Shutdown software that shuts down the PC at 8AM. Using this I have even downloaded 65-70GB in a month without paying any extra bill.
Alternative Solution: Grab a table clock which has the most irritating & "ghadi ko tod do feeling wala" alarm and set it to 7:45 AM while you sleep