Would guess that "Caesar" is a nickname for the IRS --- "render unto Caesar the the things that are Caesar's" --- so assuming a 50 percent tax rate there (probably includes local, state and federal). Looks like Garcia himself got about a third of the gross (before taxes). If I remember right, John Kahn was paid more than the other band members were, and the rest would have gone to expenses -- per diems, tour bus, hotel, Parish, sound/light guy. And, of course, stash. For most of his touring years, Garcia only cared that he had his guitar and cigarettes, it was only later on that the GD became incapable of staying anyplace other than the Four Seasons.
In those days things were done on a cash basis. There were stories about Rock cashing in plane tickets then charging new ones, or charging hotels to credit cards instead of using the funds the promoter provided, to feed the habit. Things get weird on the road. My favorite was the story about Waylon Jennings and his band playing poker with their bus driver and losing steadily to him for years, to the point where the band was fundamentally working for the bus driver.