When is the last opportunity to recycle a rune during combat to trigger Sivir, Battle Mistress's ability to create a gold token, and can you recycle runes without paying for anything?

Ruling: You can recycle a rune whenever you have priority (even without paying for anything) because recycling is a reaction ability that adds power to your pool. During combat, the last opportunity is when you have focus after your opponent's chain resolves, when your opponent passes focus to you, or after the initial chain if you are the attacker.

Sequence:

• Sivir's trigger occurs when an enemy dies (even if she's already exhausted)
• In response to her trigger, you can recycle a rune
• Then exhaust Sivir to create a gold token
• When her trigger resolves, she readies

Nuances:

• You need priority to use reaction abilities like recycling
• Deathknell or other triggers can create additional opportunities to recycle
• Recycling a rune adds power to your pool (like floating mana in MTG)
• If an ability specifically says "spend" the power (like Sivir Mercenary), you must actually use the power for a cost, not just recycle