Update product price via AJAX?

I have a Select field with a pricing rule for products in a certain category. When you choose an certain option it adds on ￡3. When the option is selected, how can I get the price shown on the product page to update to the new price? At the moment no matter what is selected in the drop-down, the product price stays the same. It is only when you add it to the cart and go into the cart, then you see the new price. I suppose this would require the price to be updated via ajax? Can this be done? Thanks