Personally, I would look into Pre-purchasing the CD. It's $99 to pre-purchase ($125 if you wait 'til after you've loaded your pictures). This works out to be a good deal if you have A LOT of pictures taken.
On the Definitive Guide site it lists locations where photographers are generally found...keep an eye out for them and for others all around the parks. The more pics you have taken, the more the CD is worth it.
We pre-purchased our CD and ended up with over 300 pictures from our recent trip.
Other than that, I don't know of any other discounts. You might see if there is anything on Mousesavers.com as well.