I bought a copy of Teratologue and found it an amazing source, both of insight and ispiration, as Trailblazer was! Terrific work guys!
I have an en-passant question for you, about monster customization/building from scratch: what about spell like abilities cost in CR points? I've been searching for that entry in table D-603 but to no use... Do I have to use gaze attack or breath weapon even if it's a totally different power (eg. mirror image, wind wall, ecc)? And what about their usage (3/rest, always on, at will etc)?
Thanks, and keep up this excellent work, I hope there will be some new releases soon!
Short answer: If it's just one or two abilities, hand-wave it.
Long answer: I always make it a point to give a shout-out to UpperKrust (Craig Cochrane) when I get into the nuts and bolts of CR valuation-- his work was the foundation of everything I do. UK values spell-like abilities at caster level x spell level x .005. It can take a lot to move the needle (hence the short answer). With regards to uses per day, anything at 5/day or more is valued at 100% (because a monster's "day" typically ends at "one encounter vs. the heroes") and you deduct 20% per use less: 4/day = 80%, 3/day = 60%, etc.
A second reason for the hand-wave is that, in the context of a creature of an appropriate level/CR to use a spell-like ability, one ability here or there (just like one feat here or there) is not going to significantly increase CR. Stick within the framework of the Spine. A creature that is CR1 should be able to use 1st level spells; CR3 gets 2nd level spells; all the way up to CR17 for 9th level spells. Of course you
can break out of that framework (you're the DM, you can do whatever you want) but it is the framework of the Spine that keeps the game relatable and predictable for the players.
I've always been amazed that the Cockatrice is CR3. The damn thing has a save-or-die ability-- a faster death even than a Shadow; a faster death than an Ogre's club. But even in the Cockatrice's case there's usually some player-knowledge working as the safety net. "That's a cockatrice! Run!"
If the players come across a custom monster who has dangerous spell-like abilities well outside the expected framework of the Spine... you might be a rat-bastard.
Back to my point: Adding +1 CR of spell-like abilities to a CR1 creature is doubling its CR (and dangerously bad design...). Adding +1 CR to a CR20 creature is a lot less to worry about. Of course if you do hand-wave it, and the ability proves troublesome for the players, do make sure you don't hand-wave the XP they would get from +1 CR.
One last point on this long-winded response: Remember that a creature only gets one action per turn, no matter how many abilities you give it. By the time you get to the 4th or 5th ability, you're not really getting full value on your money-- assuming your combats are fairly straightforward, you're pushing the limits of a typical monster's combat longevity.