Tracking the number of reviews written for each iPhone application sold in the iTunes App store won't tell you how many times that application has been purchased and downloaded. It won't reveal that apps' volume writes Medialet's David Hill. But Hill contends tracking the number of reviews users give apps will give you a sense of each app's "relative volume" — the app's approximate share of of the App stores' overall volume. Multiply the number of an app's review against the app's price and Hill says you get an approximation of its revenue, or at least its "relative revenue," which is good enough for making comparisons. Doing this math, Hill worked up the chart above. What's Hill's chart reveal? That there's riches in niches. Check out ForeFlight mobile, an app for airline pilots that costs 70 bucks a pop, earning more more revenue than any other app but one.