IMO, The values in signings today fall into one of 2 categories.
1) It's worth paying it if you have a specific item that is important to you.
As an example, I paid $325 for Aaron to sign this program a few years ago to complete it. Personally, I think $325 is crazy for Aaron, but it was the last one I needed on it.
2)The second scenario I see is for people who need some something signed to complete a set(like cards). Sure, you might find one much cheaper on ebay, but the less common the set the less likely you'll find it. Same applies for say a ball with a specific inscription that is off the beaten path.
If it's not something you are likely to find elsewhere, It might be worth spending the money so you can enjoy it. I would never pay the prices for something generic.