Assuming that you're connected to a cellular carrier, it's free. (It may need to update the phone, and that could end up costing you a fortune if you're using mobile data). The apps aren't all free, though - look at the button. "Install" or "Installed" means that the basic app is free (there may be functions that you add on by paying), but if there's a price on the button, that's how much the app costs do install. (The author decides whether the app is free or not.)