You should be able to add an OptionCost field to your option table and then add that with the add to cart when you add the item. This is something we can help with in a premier support ticket.
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.