I think there are 2 perspectives to look at this type of subject.
The first is the value of getting him, or any player really, on an item that you personally want. This argument throws money out the window as if the item is important to you, you might pay ridiculous amounts because of that personal attachment. Paying $450 for Griffey, or $600 for Jeter, etc becomes an emotional choice. As Jonathan said, he wants to add players he enjoyed watching. Personally I have "overpaid" for autos for this reason and see nothing wrong with it.
The second is the question of intrinsic and/or resale value. What I mean is how will adding a sig to an item change the sale price of that item should you go to sell it in the future. Using Griffey as an example, IMO, there are some situations where adding his sig to a 500 hr ball would be worth it and some that are not. From a financial value standpoint, a 500 HR ball with 12 sigs would certainly sell for more than one with 11, but there is no way in my mind that it would be $450 more regardless of who the extra HOFer is except for perhaps Mantle and Williams. IMO, I just don't think adding Griffey to a ball with 11 signatures will add $450 to the selling price. On the other hand, adding him to a ball with 23, to complete it, may very well push the price significantly higher than the $450 paid for Griffey.
Steiner is a great example to use. Most of their items can be purchased on ebay for significantly less than their original selling price. Many can be had for half. Going back to my first point, sending something that has personal meaning to you is judged on an entirely different scale. Paying $600 for Jeter to signed the foul ball you caught may be worth every penny. Paying $600 for a generic signed ball that you hope to resell at some point is not.
Here are some times I "overpaid."
I paid $375 to add Aaron as the final sig to this magazine
I paid $800 to add Ali to this magazine. (Yes. That tiny sig above his photo)
Would I do both again? Yep.