I assume it is a factor of the players or agents setting prices for appearances.
I noticed Julio Cruz is a VIP package guest, but only $15 otherwise. Since he apparently lives in Wa, it would cost money to fly out and stay in a hotel (if needed). I assume Cruz and the others don't pay for that stuff as it is probably covered in the signing agreement.
I met Cruz when he was a sidebar to a Franklin Gutierrez signing in my area. Cruz apparently only lives a few minutes from the shop and appeared for free, signing for free as well. It was somewhat sad when I showed up and the line wound around the shop and outside the door. There were maybe 50 people there, waiting. Gutierrez was late for the start of the event but Cruz was already inside. The shop asked if anyone was here just for Cruz (as Gutierrez was $15). I was the only one, so I went straight to the front. Cruz signed everything I had (about 15 autos) and thanked me for coming and remembering him! I suspect he signed many autos that night, but probably only because he was there, free and offering. This was the Seattle area too, where the man played the best years of his career!
I miss those days, when more players would do that, but I suspect that the resale aspect of autographs has soured many of the players willing to sign for free only to see those free autos resold on ebay!