17-18 cents (in Spain) is nowhere cheap. It's 60 cents if I want to SMS my daughter in the States. I'm actually considering buying both of us Blackberries for that very reason.
I could (sort of) do BBM with iPhone push notifications and a universal messaging app like IM+. Unfortunately, I can see my battery life drain away (literally!) while running IM+.
Not all teens are price sensitive of course, but those who are will get buried by outrageous SMS fees