Invoke-for-Effect/Compels should be working the same way as tags: spend a Fate Point, unless circumstances indicate that the tag should be free.
Note that a "free tag" for discovering or placing an Aspect can either be a +2 to a roll, a reroll, or an Invoke For Effect (I didn't get this last one for a long time).
The "disagreement" was settled by Fred Hicks awhile back (but people refused to accept it even at the time). He explains that usually, an Invoke for Effect is something that *helps* the player invoking it. However, when the Invoke for Effect ends up asking to do the same thing as a Compel (clearly causing a direct complication for the target), then the player invoking the Aspect still gets to do it for free, but at that point, the GM takes over, runs it as if it had been a Compel, and brokers a Fate Point to the NPC who was being Compelled by the Invoke for Effect.
What I'm more interested in is how to run AoE Aspects which allow a free tag, and I think that the Spray option is best for a Zonewide Maneuver placing a free-taggable Aspect.
What it sounds like folks want out of this slick spell, though, is a block on all actions.
My question, then, is this:
You can cast an Evocation Block that makes it either A) harder for one target to do any action (much like a grapple), or B) harder for everybody in the area to do one type of action (presumably to the caster, like a Veil, which is a Perception Block against perceiving the target), and neither of them require you spend two shifts to affect a whole Zone.
With this in mind, then:
1) is there a difference between:
A) a Veil (Perception Block) making it hard for everyone in that and surrounding Zones to perceive the target, and
B) a Grease Slick (Movement Block, or Attack Block, as you decide) making it hard for everyone in that Zone to perform that one action (movement or attack)?
If no, then it should be fair in the rules to make a Zonewide Evocation Block that makes it harder for all the targets in the area to do any action.
If yes, then you probably want to make it a ZoneWide effect simply to effect everybody with the single action Block. *THEN* the question is... how many shifts do you want to charge to make it Zonewide *and* a Block against all actions?