Cojibo, COB signed for free. Just grab a Mayhem Revolver magazine when you enter the venue and they will sign. I brought a CD jacket and they signed it. One problem in Chicago both Bodom and Amon Amarth were signing at 3 PM (I think or at least overlapping times. Amon Amarth had their own mini poster that they signed for free too. So did Mastodon at 4:30 which is very cool considering they had a separate paid meet and greet. MH is not doing a signing this Mayhem. John 5 signed for Zombie but I did not go and I believe 5FDP had a signing too but did not go--I believe both of theirs required purchases but don't know.
All the other bands from side stages had signings (I think) but I missed them to see bands. Hope this helps