pay vs cost

Different things — worth separating, because the word "cost" only matters for playing cards and abilities, while "Pay" is a limited action.

Cost (Rule 204) is what you must give up to finalize a card or ability: base cost, additional costs ("as an additional cost"), costs within instructions ("[do X] to [do Y]"), and applied costs ("Opponents must pay..."). Paying a cost completes the thing, and per Rule 203.2 a cost is still considered paid even if the associated game action gets replaced. Also, when a card text says "costs [X] or more," it only ever checks the printed/copied cost — never additional or Repeat costs (FAQ #3673, Rule 206).

Pay (Rule 444) is a Limited Action: removing Energy or Power from your Rune Pool, which you can only do when a game effect directs you to (Rule 444.3). The key trap is Rule 205 — if an instruction to pay resources has no linked effect ("you may pay [4][C]. If you do, kill a unit here"), that is not a Cost at all. It's a game action, and the later instruction checks whether you actually did it, so a replacement effect that swallows the payment means "if you do" fails.

Practical upshot: cost-based templates care about the attempt, "if you do" templates care about the outcome. If you meant a specific card interaction, name the card and I'll rule on that.