There is no relationship between GRE score and scholarship amount. You will be judged on your whole application, not specifically GRE score. The scholarship amount depends on the school and also on the advisor, if you're doing a research assistantship. The rough amount would be written in the application materials. If you can get a full scholarship, usually it is enough for most monthly expenses, and most of the time, you can save a bit too. My rough guess is that the amount varies from $1000-1500/month. It might be slightly more or less.
