In CBM it's only 3N, and at that price, I found it extremely cost effective for my LA Pangaea Pans & Panic Apostates (330 & 320 gold, respectively; neither are sacred).
The spell turned 22 gold/turn units into 0 gold/turn animals. The major drawbacks are the chance of feeblemind (I guesstimate that's around 10-15%) and the loss of slots (most of the animals only have misc slots). However, I did once get a flying, fire-breathing chimera that could self-cast Invulnerability & Personal Regeneration; another time, I got a Winged Monkey with full slots. More typically, you get various animals with interesting abilities (like mountain-survival cave drakes, flying wyverns, or stealthy moose).
Plus, when your unit becomes feebleminded, it always transforms into a Foul Spawn of some variety, which can be not-so-terrible...
