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.
2/1/2014 The amount of mana you spent to cast this creature is usually equal to its converted mana cost. However, you also include any additional costs you pay, including the cost imposed for casting your commander from the command zone.
2/1/2014 You can’t choose to pay extra mana to cast a creature spell unless something instructs you to.
2/1/2014 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.