Frozen yogurt

Well, actually I wonder how to make frozen yogurt.
I've tried out several frozen yogurt and I always love how the frozen yogurt melt on my tounge :)
Since, frozen yogurt is very popular now ( i dun know how it is in melb, but yah in my city).
they sell it in quite high price. I wonder what makes it expensive. Is it because they are rare in my city, or simply because it is hard to make?
If anyone know how to make frozen yogurt, please let me know ok? :)