OMERUS is a simple app that lets you use the iPad as a math workbook. You can easily manipulate numbers using the intuitive interface. You can also select any number with a single click instead of typing it out again.
Theres a log of your previous calculations that allows you to label each calculation with a text description. This log can be shared via email as a PDF file.
The app is perfect for doing cost calculations, basic accounting or simple arithmetic.