SIM card for SMS messaging application. Just pay per text (SMS) rather than monthly fee?
This is prepaid plan. However you must bay some credits (even if you have a few left) every month (that maybe different period in various carriers) to keep Your account active. But You can spend all credits for sms so technicality there isn't any "monthly fee".

But I think much cheaper and reliable solution would be using some Internet sms gateway for example http://www.clickatell.com/.