Most mobile phone service providers now offer an unlimited text-messaging plan, which is a good place to start so you're not paying for every message she sends. Check your provider's web site for this option and research the rates on "digital" plans to get a handle on the cost of your daughter surfing the Internet via her phone if it's web-enabled. You can also consider getting your daughter a pre-paid phone so you can set a cap on her overall cell phone budget. To compare rate plans for major mobile providers, check out sites like My Rateplan and Let's Talk.