$15 a month. That's the number I was looking for but couldn't find until after creating an account and trying to add a phone number. Then a popup came for my credit card number (which I don't have) and finally, in the confirmation button, was the price.

Edit: Oh, it's in the FAQ. I checked all other pages except that one (seemed logical to be under either home or features, or perhaps policies). My bad.

Edit2: Giving the price a second thought, is this a normal fee in the US? Our landline costs just over 4 euros and the cheapest mobile subscription I've been able to find is 3.50 a month.