Purina Pro Plan and Purina One Smartblend have an almost identical ingredient list and guaranteed analysis on their products, but you pay double the price for pro plan.
Royal Canin is $124 a bag and doesn't contain any real meat (lean animal protein)! It contains mostly corn (hard to digest and high on the glycemic index as a carbohydrate) and chicken by product meal (very little meat, with ground up bone, skin, intestines, kidneys, feet, eggs etc).
Hills Science Diet is $97 a bag yet again, no real meat in the ingredient list at all! (low animal protein, high plant protein).
Obviously different formulas on small breed, senior, adult, puppy etc so ingredients will vary, but the majority are the same. You are paying for a brand name, not high quality dog food.
*Edited for clarity as my 2am rant is lacking
Chicken meal is ground up chicken meat, bones, and skin, but there is no measuring of percentages of each, the plant that renders the meal does it in high volume, so their is no way to know what percentage of what you are getting.
Chicken by product is the bone, feet, intestines, egg all rendered together, and again, there is no guarantee on what amount of each gets into the dog food.
Chicken by product meal is just every part of the chicken all ground up together.
Having a meat listed as the first ingredient ensures the dog food contains a higher percentage of animal protein, and having by product or meal afterwards is not harmful or bad, but they are not substitutes to actual muscle proteins.