It's all about the Benjamins. There's a burgeoning market in graded signed cards, especially rookie cards and rare cards. The numbers are truly staggering in some cases. Case in point, this 2013 Upper Deck PMG Tyson with autograph:
$58,800.00 at Goldin.
The base card is rare: UD made 125 numbered sets and gave them to employees as a bonus. About 1-2 a year are sold. Until a few years ago it would run you about $500 for a Tyson. Even now, you can get the card for under $5K. Get one and have it signed, it has a massive multiplier, even without dual grading. It makes it worthwhile to bite the bullet and pay up. There are lots of cards like that. Koufax's RC sells for a fortune dual-graded, so he now charges over $3K to sign it, and he sells out every time he offers a private signing. Jabbar's handlers justify four-figure up-charges for signing his RC based on what they think PSA will grade it.
What's even worse are reports that some guys won't sign their RCs or specific issues any more. This used to be a rare thing; now it is increasingly commonplace. I suspect they are trying to create a perception of rarity over certain cards that they can sign and give to family members as investments.
Then there are the rare items that you just have to get signed at whatever the cost because they are (nearly) irreplaceable...and you think they will sell for a bundle if offered. I am going to spend well into three figures to get Dr. J to sign this 1971 Virginia Squires team issue at the National:
An impossible item signed by a legendary player, again, cha-ching if I want to sell it.