As far as I know there is no discount for SS. I wish though. I've put through about 80 books for SS.
But Kev is right, there is so much time and effort involved with getting the signatures. If you have ever been to a large convention, you know that waiting 1-2 hours is common for getting autographs (sometimes longer). And meeting the artist at a specified time is next to impossible for the artist, the dealer and a CGC rep to co-ordinate.
$25 may seem like a lot, but it is worth it, particularly if you are able to get a sketch on the book as well (it's rare but it happens if you pay a little extra to the artist).
I hope that helps.