Oh goodness- considering what a service you are providing... that is more than reasonable, esp compared to the 'percentage' that lawyers tend to charge. The work you do for people involves a LOT. These cases cannot be won by just making a phone call or two. It's a complicated system to take on and you provide a service that is incredibly valuable and needed... and you should not be expected to go broke doing it.
After all, after people win their cases they will be furnished with a means to pay their bills. It's only reasonable for these same people to understand you need to do the same, and for this to happen your organization needs to be self-supporting.
I wish you the very best!!