When I was doing the rock'n roll circuit, some years ago - 3-4 nights a week - we paid our agent a steady 15%. However, on really well paid gigs - I'm talking around £600 for 3 musicians in the 1980s - he took 20%, and we still thought of ourselves as well paid. So - a good agent, who gets you gigs of the right sort, at the right time, in the right place, at the right price, is worth a 20% cut.