Another possibility is that this much 'change' to a character's essential nature is not trivial. If you want to add a power, it's possible that person is 'obsessed' with it. Or, maybe you are the Hulk and lose some other capabilities. Maybe you lose your free will a bit if you normally would by taking that power and become something of an NPC for a time. There are lots of ways to discourage this type of "gamesmanship" that the GM has, I would say ... and it certainly seems like you should discourage it.
Well, what if flying isn't done by adding wings, but instead by giving someone a cloud they can ride on? Or it is done via making their overly large cloak work like wings? There are gray areas for many of the powers. Hmm, I SUPPOSE one could treat a flying thaumaturgy as allowing something otherwise impossible. You can fly like you were operating a helicopter without needing the helicopter. That's just granting a drive skill and this works much like tracking something you can't actually track. Might well be too cheap though.
One of the main playtesters, Rick Neal, basically said to add a power to someone you have to "take out" the target, which means as good as kill them:
1. Overcome their Stress track (3-5)
2. Overcome their Conviction (shifts equal to Conviction +5) - may be optional for a willing target
3. Fill up their Consequence slots (20 shifts)
4. then the shifts to buy whatever you needed to buy. So anything transformative is 30+ in Complexity.
Other folks think that's way too much for being able to fly, or see in the dark, or something else. But the Transformation and Disruption rules should be a good guide if you want to actually give someone a power. Another good way is to use an Item of Power (someone on the boards had a Flying Hockeystick they rode like a witch would a broomstick).
The rules explicitly say this is for a permanent change, not a temporary one. Regarding a temporary power...they don't say much at all:
"Transforming yourself is either about producing a temporary, short-term, specific spell effect with an obvious shift cost, or it’s about acquiring new powers with spent refresh (or temporarily—see page 92—using fate points and tags)."
And page 92 just says this shouldn't happen much (and that if you don't have the fate points to cover the refresh cost, you can owe the GM compels). I'll grant a lot of this stuff can be done with specific spell effects, but there is also a lot of it that can't. While requiring 2 fate points for a temporary -2 refresh cost ability is a bit expensive, we still don't know how difficult it is to work that spell at all. The guidelines they provide don't seem useful for determining this at all -- they seem more about curses, temporary aspects, and full and permanent transformations.
