Given the effect is temporary, single target, and doesn't really offer protection, just makes things harder to analyze, I'd say a pretty low casting cost is in order. Maybe 2 or 3
.
Alternatively, this may make a very nice eternal spell
(see the thread in design theory:
Eternal Spells)
E.g. Make it an eternal spell, drop it to one or 2 turn length and give it a 1 or 2
cost.
...Probably need to add a discard effect as well if there is room.
As an eternal spell this would be very cool I think.
Lastly, there should be some way to dispel the effect... maybe just have it be dispelled like invisible? If this is an eternal, that could be the discard effect.
You could word it as:
"Eternal: Retained after casting
Hide status of target card in play for 2 turns or until this card is discarded."
There should be enough room to add "or player" in the upped version as well.
Just put that it can target immaterials / untargetables in the Notes.