"if there is another Frenzy card in hand, opponent receives bonus damage (N * recruits cost of the played card / 3, N based on played card rarity (Common - 1, Uncommon - 2, Rare - 3))."
The way it's worded, I was doing the math wrong for a while. I would rephrase it either as:
"if there is another Frenzy card in hand, opponent receives bonus damage (recruits cost of the played card * N/3, N based on played card rarity (Common - 1, Uncommon - 2, Rare - 3))."
or
"if there is another Frenzy card in hand, opponent receives bonus damage (N * recruits cost of the played card, N based on played card rarity (Common - 1/3, Uncommon - 2/3, Rare - 1))."
Alternately, and this gives a different value when Uncommon (halving rather than 2/3) but may be easiest to read,
"if there is another Frenzy card in hand, opponent receives bonus damage (recruits cost of the played card / N, N based on played card rarity (Common - 3, Uncommon - 2, Rare - 1))."
As it is now, I kept thinking N was in the denominator and that the relative effects were thus reversed. In any case, it's harder to predict exact damage when dividing since you have to either know or make a guess as to when rounding will occur. This might be more math than it's worth.