* For price: πόσο κάνει; (póso kánei;) This is the most common way to ask "how much does it cost?"
* For quantity: πόσο; (póso;) This is a more general way to ask "how much," referring to amount, quantity, or extent. You could use this asking "How much sugar?" or "How much time?"
Therefore, there isn't one single translation. You need to choose the phrase that best fits the situation.