Essentially, all Void cards can be considered "free" cards, since there is, in fact, no restriction to playing them from your hand or using their abilities. If that is so, I suggest a 1.5-3X multiplier to any balancing costs, since 3
is equivalent to a single elemental quanta, and the opponent will be getting random quanta. 1.5 because you're paying half of its value to play it without any restrictions, while it could be argued that it should be 3 so that the opponent will be competitively kept on the same track as you, since this would essentially be a rush deck strategy.