The answer depends on the goal of the purchase. Fun? Collection? Make money? If it is make money, forget signings and go with an already-signed item that is certified. Koufax has been signing for decades, so there is plenty to choose from and be choosy about, and it sells at a discount to signing fees.
Interesting question, though, about how amazing these costs have gotten. I have only paid for one autograph signing in the last several years and it was strictly to get a rare item signed, a 1971 Dr. J rookie year team issue:
As for Koufax charging what he can get, hey, hate the game, not the player. if the market decides his charge has reached its ceiling, we will know.