It's 25%, subject to availability. They only offer them if there is a reasonable chance that the room will not be used for points, it comes from member inventory. Housekeeping follows member rules.
Each year DVC negotiates with DCL for a contract of cash prices for members using points to book combined with cash. At least one adult must booked with DVC points in a stateroom. The remainder occupants will be charged the cash price contracted between the two.
Although it is a reasonable, discounted price, it is not the lowest priced by any means. Early Booking Savings (for those booking totally with cash) is your best discounted rate.
If you do choose to book with MS, you will pay the $75 fee to place the reservation, and the cash prices for the other occupants will be due in full at time of booking.
Depending upon how many points you own, what you desire to do with them, and how much you are willing to spend, you may or may not want to book through DVC. I have on a couple of my cruises and I have never regretted it.