re: How much would this cost?

It’s because signature collecting is highly niche. Say someone is looking for a Joe Burrow signed ball. The person may only want the one signature and not a ball signed by others, especially from different years.
So the demand and value of the item go down. Single signature items are the best. Or people who played together.

That’s if you want higher value anyway. If you are getting it signed for yourself and never sell. It doesn’t really matter.