welcome to the forums gamingdoneright
I'd suggest the first step is you look at the source code to the page they link. That uses inline Java code to define the price. When then user has completed the form it then runs a short script which is passed the variables ( form name='signup' method='post' action='step2.php' ) which can send the price to paypal.
Which part is it that you are struggling with ? All the changing of the price ( which they have done in Java), the passing of that variable to the step2.php script or passing the info to paypal ?