Well Becca, I would say that this is a really hard question. Regardless of the money associated with having a cell phone, there are two sides you have to consider.
1. The potential negative influence on the health (if used too much).
2. Keeping up with other kids as most of them already have one and your kid is going to feel usually very bad not having a mobile as well.
To be honest I even try not to use microwave or Bluetooth as they work on a similar frequency as cell phones or Wireless networks.
The fact is that in general you don't have to worry about that too much if you use these things not too often. That means that as children are still growing and forming their immunity and everything, I would buy them a mobile phone, however only for keeping up with the technology and friends and for emergency calls only. If you've ever experienced some sort of headache after calling with somebody a little bit longer than usually, imagine what impact it can have on a child.
And it's not only with these waves but everything. Their immunity against any negative influences is a lot weaker in comparison to an adult.