What percent of the cost should teenagers (ages 13-17) pay for their own cell phone and service?

A cell phone or not? This is the question that parents sometimes have to decide. Do I give my teenager a phone, or do I make them earn some money for it? 
What do you think?
Cell phones, or mobile phones, are becoming more popular around the world as a preferred form of communication, but can you find a cheap calling plan that will fit your budget? What do you look for when deciding on a cell phone or a phone carrier (e.g., cost of phone, number of minutes a month, cancellation fees, etc.)? What types of international calling cards can be used with such phones?

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