The point cost of trading out, or even the Disney Collection reservations, is clearly stated as subject to change, as Doc points out.
That's why you hear on this board very often, that DVC is a good vacation investment if you plan on going to WDW (or HH or VB) and staying at a DVC resort. Anything other than that is not price protected, or even guaranteed to be around, if I'm not mistaken. So you will often read here that DVC is not a very good investment if you're planning on trading out often.
Nothing wrong with using points for Disney cruises, the Polynesian, or trading out to other timeshares. It is just the opinion of many that it's not the optimum use of the DVC membership.
Even with DVC resorts, we are not price protected as the annual maintenance fees can fluctuate. But we are protected there in that they can only go up in response to actual costs rising.