Partially inspired by this question I would like to know exactly how ona'a (over/underchanging by more than 1/6th) is determined. For example tap water is free and in many parts of the united states is 100% safe to drink. Yet people pay more than $1 for bottled water. Why? I would chalk it up to marketing. Does that mean that I as an orthodox Jew cannot sell or buy a bottle of water since I would be duping the consumer?
In short, what are the parameters for determining if one person is violating ona'a?
Note: please don't get bogged down on the example; feel free to provide a better one that demonstrates the question.