How much and who to pay
It all depends. You have to find out what the organizer and videographer agreed upon in detail. It is possible that the organizer holds all rights even to unpublished material.

The amount you pay is dependent on how much time effort and maybe investment in equipment was taken to produce the original material. But your budget will set a limit anyway. Assuming you can track down the videographer talk to him on the phone and try to find out about their efforts, then offer a higher or lower percentage of your limit.