The following article outlines the costs to send a text message from OnlineGiving.org or Ministry Platform.
Text messages cost $.0075 to send per user. So if around 100 subscribers and each message is costing .0075, then it will costs you $0.75.
However, remember that text messages have a certain length limit While it may look like one text message on your phone, it could technically be multiple messages. Here’s a tool to figure out how long your text message actually is. https://chadselph.github.io/smssplit/ (select ucs2 on the menu).