Cheap vs Expensive. Which one is better?

Does the quality depends on the price? I'm surveying products some affordable and some expensive. So, how do we know this product is really good and worked?

Sometimes I wonder, if the cheap goods have low quality, why do the company keep selling it? Does it bring profits to them?

I purchased certain quite expensive and certain affordable products depends on how my body reacting into it.

Expensive products sure good but my it hurts my wallet too.