I noticed that quite a few of the monsters have Powers, but that there is no AEP cost listed for their use. Is this up to the Lejend Master to determine?
The AEPs cost for Power use are not listed for a simple reason. In most confrontations, the creature in question will not come anywhere near using up his Extraordinary energies. I assumed 1 point per use. Put another way, the LM can assume that the Powers are at no measurable cost to the creature in question, as a 1 AEP cost per Power use is virtually no cost. Combat will not last long enough to make a meaningful drain of AEPs to such creatures. The Lejend Master has full discretion, of course, to assign AEP cost to any Power. High AEP cost, say from 5 to 10 points per use, would indicate a monster of less potency than presented in the BEASTS OF LEJEND book, of course. That is a good way to enable less-powerful Avatars to face such a creature.