Yes, pay for every character in the horde (another reason to break hordes into smaller groups).
However, as a house rule, I suggest setting the cost at 1 for every interruption, regardless of target category/type or number of creatures in a group; the cost is for the action, not for the creature. This was something determined some time after Mutant Chronicles was completed, so it's not handled that way in the rulebook.
Game Development - 2D20 System
System Design - Star Trek Adventures
Rules questions and playtest feedback to
nathan@modiphius.com