Multiple Widgets On Page - Updating Fields?

I have a page, that will contain up to a couple hundred different widgets on it, that an individual will be able to browse through and choose the size and quantity of the ones they want. Each widget will have form fields associated with it: "size" (small, medium, large) and "quantity", with a "total cost" field underneath that. As the individual looks through the widgets, choosing the size and quantity of the ones they want, I'd like for the "total cost" field to be updated under each widget. I'd also like a "total order cost" field that sums up all the costs of the widgets they want.

I need the script to basically say:
If size = small, then multiply $1 by the quantity.
If size = medium, then multiply $2 by the quantity.
If size = large, then multiply $3 by the quantity.