Prossh, Skyraider of Kher

When you cast this spell, create X 0/1 red Kobold creature tokens named Kobolds of Kher Keep, where X is the amount of mana spent to cast it.
Flying
Sacrifice another creature: Prossh gets +1/+0 until end of turn.

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When you cast this spell, create X 0/1 red Kobold creature tokens named Kobolds of Kher Keep, where X is the amount of mana spent to cast it.
Flying
Sacrifice another creature: Prossh, Skyraider of Kher gets +1/+0 until end of turn.

Masters 25

Flying
When you cast Prossh, Skyraider of Kher, create X 0/1 red Kobold creature tokens named Kobolds of Kher Keep, where X is the amount of mana spent to cast Prossh.
Sacrifice another creature: Prossh gets +1/+0 until end of turn.

The amount of mana you spent to cast this creature is usually equal to its mana value. However, you also include any additional costs you pay, including the commander tax.

The first ability triggers when you cast Prossh, not when it enters the battlefield. This means that it will resolve before Prossh, so any Kobold tokens it creates will be put onto the battlefield before Prossh. If Prossh enters the battlefield without being cast, the ability will not trigger.

You can't choose to pay extra mana to cast a creature spell unless something instructs you to.