DVC exchange rates for Adventures by Disney, Disney Cruises, and other external exchanges aren't usually optimal. These exchanges give DVC members alternatives for using their points on non-WDW/
Disneyland vacations, but they come at a premium. The members aren't getting the same financial savings on these vacations that they get when using their points at DVC locations. In fact, it's quite the opposite. The value of the points used for one of these vacations is usually higher than the cash price of the vacation. Therefore, while renting makes for a great deal for a stay at a DVC resort, it's usually cheaper to pay Disney the cash directly when it comes to cruises or Adventures.
Is it actually a DVC member-only event, or is it an event that just isn't open for booking on cash yet? If the latter, then you'd probably be better off waiting.