Some confusion about unlimited text plans

I have my iPhone on my own personal account and am just getting the 200 text messages for the additional $20 per month. I have been told by a couple of people that there isn't any way to bundle packages together with the iPhone, that is, if you want unlimited texting you have to pay $40 per month for each phone. My wife has four phones through her plan, including our two college daughters. She pays an addition $30 per month I believe for unlimited texting on all four phones. I have been told by a couple of other people that there is some way to create unlimited texting without spending the full $40 per phone. Right now my daughters have Samsung sliders, but I might consider eventually putting them into an iPhone is I don't have to spend an additional $80 per month to cover their extensive texting habits. Anyone??