Pricing largely depends on location.

Certainly in the UK, and most of europe, receiving SMS is free. Sending them might be 10-12p per message, but in practice contracts allow "unlimited SMS(sending)".

That's one of the reasons why I'd be hard pressed to use an application on a phone for sending messages - I already have that with SMS, and I don't need to know usernames, etc, I just use the contact number I already have stored.