Definitely not a discount. In fact, you'll probably be paying more than you need to if you use points for a cruise. Check out the cash price and compare to points. I value points at about $10 each as that is what I could get for them in a transfer to another member.
If things haven't changed since the last time I did this same thing, you'll have to pay the rate negotiated between DVC and Disney Cruise Line, regardless of any better deal you find elsewhere.