I think most people would consider 50% reasonable, and we know it can be done because several agencies are doing it, e.g., pond5, Alamy, featurepics. Some new agencies have paid less and still attracted contributors, but they had sales. i'm thinking Canva here - they pay 35% but concentrated on generating customers first. Many contributors will submit a few hundred images to see what will happen, but will stop if there are no sales. A good explanation of what makes you unique for customers and how you plan to attract them would help.