jquery script customisation

Hi I need a short jquery snippet to do some simple calculation. I provided a simple head start as attachment, please have a look at this before continue reading. ATM the three input fields (class="quantity") sum up in the div with the class="total". What I need: Before summing up the values each user input in class="quantity" should run through a unique calculation. It should be automatically multiplied with a unique value. The whole function should run on keyup. Thanks