Cost based shipping charges would have to be added using eCart and Dreamweaver.
We could help you with something like this in a premium support incident that you could purchase from the Support section of the webassist site.
It would involve using eCart to set up your shipping rules to calculate the shipping price based on the cost and then making sure that the shipping total is properly passed to paypal during the checkout process (most likely this would be automatic once the charge is set up).