To send a mobile text message to a US phone number costs 1 text credit (i.e. sending a text message to 100 phones costs 100 credits).
No charge applies if people text your mobile keyword for their initial mobile signup or if they verify their phone by replying with OK after entering their number on your Online Signup Page. Also, you do NOT get charged if people text HELP or STOP.

However, please note that if they respond to any survey or contest or text your keyword more than once, each reply or keyword text will cost you half a credit (0.5 text credits).