Tea-conomics

I've just bought 80 fairtrade teabags for 28p - Sainsbury's Basics range. So, how does that work then? If you can sell fairtrade tea for 28p - including the cost of processing, packaging, shipping etc. - just how much does the grower get for fairtrade tea? It doesn't seem like nearly enough. Do all fairtrade brands pay their growers the same, or is some fairtrade fairer than others? And if they all pay the same, how do some brands justify charging 10 times as much or even more?