If the attacking creature or creature that entered the battlefield attacking isn't on the battlefield as the triggered ability resolves, use its mana value from when it was last on the battlefield. (If it's on the battlefield but is no longer attacking, use its current mana value.)

You decide whether to pay {1}{R} as the triggered ability resolves.

Putting a creature onto the battlefield attacking as the ability resolves will cause the ability to trigger again. This instance of the ability will consider the mana value of the new creature that entered the battlefield attacking.