How much do you get when you're going to school on the GI Bill?

Depends on a number of factors - your rank at the time of separation, if using the post-9/11 GI Bill, how much time in service you had after 9/11, whether you're using the Montgomery GI Bill or post-9/11 GI Bill, cost of living in your area, whether you're enrolled as a full-time student (12 credit hours or more per semester), etc.