They range from company to company. You have to pay so much for the phone and a start-up fee. Then they range in price depending on usage a month. They can range from about 10,000 to 40,000 on average. If you plan on using it a lot and whatnot, a plan might be nice.
I opted to go with a two year contract with a smart phone. It's only 55,000 a month for 300 minutes, 200 texts, and unlimited internet. I think it is worth it personally.