build in javascript caculation into form

Hi, I;m lookign for someone that can build in a javascript calculation into a precreated form. Some of the thing the javascript needs to do are: - to multiply an price with an amount the costumer want's to buy, and check if the minimum amount is bought. - based on if an certain textarea is filled with text, add up the precalculated amount with a certain price. - Check if the 2 or only 1 of the textarea (in the form) are filled with text. if 1 an other ammount has to be calculated than if there are 2 filled. - Check if 2 select boxes have the same value, if the value differs then an amount has to be add up to the total. The form is an multipage form, on request a link is send.