I am using jQuery to calculate prices. Here is a small snippet of my JavaScript:
// Prices: radio
var curLam = \"gloss\";
$(\"input[name=lamination]\").click
Generally speaking, you shouldn't return the selected price to the server. HTML forms can be easily faked. It is far safer to send the user's choice back to the server, which also knows how much it should cost.
Basically Javascript validation (which this essentially is) is convenient but shouldn't be trusted. You already have an input for selecting the finish. Just send that back to the server. That way your site will still work (or work better) if the user has Javascript disabled.
You could put the ID of the product into an input element and then when you submit into the shopping cart, then the value should be in your php $_POST variables.
There you can check the price and other information straight from your database.