Depends on how you write it. If the app uses a web service for sending SMS messages in bulk, it may be cheaper. If the app uses the phone's carrier to send it, then the price will be the same.
Umm have you thought about just emailing the the SMS to the phone, still have data rates that apply.. I know here in the states I can text my friend via email and have a two way conversation with them on their cell and me in gmail.