Just to add that the 10% commission paid to the agency by Disney is after all the taxes (sales tax on tickets, rooms tax) are backed out, so it's not a straight 10% of the entire booking but a little less. Different host agencies will have different commission splits, so make that part of your inquiry to a host agency.
Disney planning often takes time, and your clients will have a lot of questions. Some will make their own ADRs, others want you to make those for them, as well as asking you for dining recommendations, park and ride info, etc. My point is that taken on a one by one basis the actual monetary commission made on a Disney trip is low compared to the time involved that you spend on it. Large agencies make this up in their volume of bookings, but a small business can't get sustained high booking volume. Just adjust your expectations accordingly.