Javascript, AJAX sum total display box from selections

This should be a very simple project for someone who knows Javascript or AJAX. Basically, I have an photography website that sells prints to clients. When a client clicks on a print it will allow the client to choose the size and a number of different options. The size is a drop down selection menu that is generated from a mysql query. All of the options are created from another php file that is updated dynamically, depending on what options the admin wants to create. For example, in the admin I have created the following options: Color (Sepia, Black and White), Paper (Metallic, Black and White), and Finish (Lustre, Glossy)...I have the ability to add more option categories and more entries into each category so whatever solution is created for the sum total box it will have to be functional when more categories and options are created on the back-end. What I need is to have a separate file created that will allow all of these selections to be added up and a sum total shown in a read only text box before they are added to the cart. I will simply do an include command in the php file to make this work.