The official member discount is 25% if available. However, check around, there maybe other promotions with higher discounts. For example, the July 5 to Sept 30 promotion is 30%. I believe there is also an AP discount of 40% out there for some dates.
You can get a DVC discount if it is available, but you book those cash stays through DVC. If you have an AP, you can maybe get a discounted stay through Disney Reservation Center. But, in all likelihood, you'll be paying rack rate.