Apart from weather apps, I have the feeling that designing a calculator is a theme that got a bit overused on Dribbble.
The challenge of this fourth day was to design a calculator that would feel different, yet very easy to use. Its goal is simple : you want to buy something, usually expensive. This calculator simply shows you how much time you will have to spend working to be able to afford it. Handy for money savers... Discouraging for the others :P