If we compare stevia with sugar, yes stevia is expensive. But if you compare other factors, stevia is 300 times sweeter than sugar. So we can say that 1 gm of stevia is equivalent to 300 gms of sugar. If you consider this factor, then stevia is cheaper than sugar.
Artificial sweeteners are also quite cheap as compared to stevia. But artificial sweeteners are bad for your health and it is not natural. So there are many side effects also. But stevia is completely natural and safe for everyone.