This question really has two questions in the same post: What is the relationship between zone density and wealth?
Just to be very clear, the two questions are 1. What is the relationship between high density and wealth? 2. How do I get high density industrial zones?
Additionally, question #2 might be better addressed by a more general question, "How do I increase zone density".
It seems like it would be better to have this question split up, but I'm not sure of what specific action I should take. What should I do?