I can't decide on how to word this, nor what a fair cost would be. :(

Does "Until end of turn, you may exile the top card of your library rather than pay the costs of abilities of creatures you control if it is the first time you would activate that ability this turn" make more sense?

"Until end of turn, each creature you control gains 'You may exile the top card of your library to activate an ability on this creature, but only once.'"?

Strange, that is awkward no matter how you say it. Maybe it shouldn't be an option? Maybe it should just activate your creatures when you play it. Is there a specific reason why you need the exile? Trying to line it up with Exile: the Setetting?

It's probably more readable that way. Notably, that wouldn't lock in the set of creatures it applies to when the sorcery resolves, which means Myr Propagator likes the revised version a whole lot more.

Of course you have things like Spawnsire of Ulamog that like it no matter how it's phrased, but that's presumably the point, and by the time you get up to Omniscience mana, even Wizards agrees that mana doesn't really matter.

It doesn't need to exile, I guess. I just want the first activation to be "free," and I wanted to give it some sort of cost.

Also nice on the Myojin like Myojin of Seeing Winds, but that needs you to have cheated them into play. Or on an animated Door to Nothingness.