One of my most popular apps right now is Project Pricing. This is an app that I wrote because I needed an easier way to figure out how to price items that I was selling on Etsy. This was my solution.
The app solves some common issues that people have when they price their own items – such as including enough to pay themselves. The simple layout lists several fields that allow the user to enter their own numbers. It also allows you to decide if you want to calculate how long it took to make the item in minutes, or hours.
It then allows you to play with the margins you want to make and lets you quickly see how that affects the overall price of the item. Since I personally use Etsy to sell items, it included an option to include Etsy fees in the cost.
This app is as educational to explaining pricing downfalls as it is useful to those that understand how to price their items.