If you have a plan with minutes and messages included then you will only be using part of your minutes or messages plan. No extra charge. I have a plan with minutes and messages included in my monthly plan, so I don't pay anything unless I use more than I am allowed. Hope it helped.
If you are using an app like textPlus, then you will be charged if the person you are texting or calling doesn't have the app. If you both have the app, you won't be charged for the texts or calls. If you are using regular carriers, you will be charged for texts and calls. See details on www.textplus.com/.