Price calculation

Hi, I need javascript or php or both to calculate price of worktop size. We need text field where customer adds price and by added number it will show price (£2.25 per 100). Then we need drop down box where customer selects from colour. Some of those colours are 25% of that total that has been calculated based on the size. (other colours have no additional cost). Please let me know posibilities and your solution. The name="" fields must be editable by us for transfering data to a shopping cart. Thanks