Xeviat already hit the nail on the head: There is no ratio. At all. It's entirely dependent on how their class levels change where they fall on the CR calculator in the DMG.
Also, bear in mind: CR, in general,
does not take versatility into account. At all. Not even a little. Seriously.
CR only cares about damage and HP, and to hit and defense insofar as those directly affect damage and HP.
So, for example, if you add casting abilities to a monster, but it has no spells that directly deal or mitigate damage... misty step, charm, see invisibility, fly, etc... the CR may even stay
exactly the same.
I said this in a different thread:
In general, CR is not really a representative of how difficult a monster is going to be to fight. I think it's mostly just an expression of whether or not the monster is able to do so much damage that a single lucky turn or two could start killing people (or has so much AC and HP that low level parties will struggle to bring him down in 5 rounds). That's why, right there in the DMG, they basically write off tons of things that add huge amounts of versatility to a monster as being irrelevant to CR... like speed, stealth, teleportation, ranged attacks, etc.
Read more:
http://www.enworld.org/forum/showth...Boring-quot-Monsters-Ogre/page6#ixzz4V7ClafOL
If you add spells and, for example, now they can Cone of Cold twice in a combat, there's a good chance that will change the damage expression in the CR calculator, so their CR will increase.
All that said, presumably your goal in adding versatility is to make the fight more challenging. So, you probably want to increase the CR regardless, to represent that. After all, you
know you're going to use the versatility, whereas for MM monsters the designers can't know how much a particular DM will do so.
In such cases, I think it makes sense to at least increase the CR by +1 or +2. That's what I generally do, at least.