A new study from Pew Research Center discovered that only 12 percent of American teens age 13-17 do not have a cell phones of any type. To make the correct call, though, do the math to be sure you are ready for far-reaching consequences. After all, it's not just a one-time purchase that parents are agreeing to, but a stiff monthly charge that could last for many years to come.

If you get your 12-year-old a plan that costs, say, $50 a month, that will set you back $4,200 though age 18. And that's not even including any ancillary costs like equipment and upgrades, repairs and app purchases. Data overages, especially if your kids are heavy video watchers, could inflict significant extra damage.

Indeed, 23 percent of households report paying much more for their kids' phone plans than they originally expected, according to a study by the National Consumers League.

That doesn't have to be the case if you are thoughtful about how your decisions will affect household finances. Here are some suggestions:

1. Start with baby steps
A basic cell with phone and texting capability can be very reasonable indeed; Sprint, for instance, offers a 'WeGo' starter phone for only $9.99 a month. There are also prepaid plans available, with varying restrictions on minutes and data, and low-cost handsets. T-Mobile, for instance, offers a $40-a-month prepaid plan with unlimited talk, text and data on its own network, and 1 GB of nationwide LTE data. With hard limits in place, parents are essentially saving themselves from any unwanted bill surprises.

2. Go for the family plan
If your kids prove responsible with their new gadgets, and aren't constantly calling or texting their buddies late into the night, then you can talk about graduating to more elaborate phones and plans. When you are all ready, every major carrier offers a version of a family share plan, like Verizon's More Everything and AT&T's Mobile Share Value.

3. Have the money talk
"The question that must always be discussed is, 'Who will pay for what?'" says Mark La Spisa, a planner with Vermillion Financial in South Barrington, Illinois. "It's critical to talk about it in advance of a child receiving their first phone."

For an 8- or 9-year-old, it is unfair to expect anything beyond a token contribution. But teens who have their own income from part-time or summer work can start chipping in to cover part of the bill.