For rebuttal, to take control of it, do I pay the power on resolution or to put it on the chain?

On resolution — not when you put it on the chain.

You play Rebuttal for its printed [1][P]. The "You may pay [A]" is a cost within an instruction paid when Rebuttal resolves, not an additional cost of playing it (FAQ #12038). When it resolves you choose: pay [A] and take control (making new choices), or decline and counter the spell.

One catch: the target spell must still be on the chain when Rebuttal resolves — if it's countered or removed first, Rebuttal fizzles and you keep your runes.