[Plugin: WordPress Simple Paypal Shopping Cart] Customising the variations to update price

I know you can do say [wp_cart:Demo Product 1:price:15:var1[Size|Small|Medium|Large]:end] to get a drop down of options, but I want the price to be updated depending on what option I choose. I am quite handy with php so I would be comfortable getting the values and updating the $price variable, but I can’t really see what is what from the plugin’s code.

So has anyone else had any luck with editing it to update the price based on options chosen in the dropdown?

http://wordpress.org/extend/plugins/wordpress-simple-paypal-shopping-cart/