java script help

[Acers]

i am looking to create a running total of the values a person selects from a dropdown list.

eg i have say 10 products.
in that each product has diff points to it.
all the 10 are accessible to the user via a dropdown menu in which he can select say 3 products.

so basically there are are 3 dropdown menu selections. every user has to select 3 products from the total available 10 via the dropdown.

I want to show as a box at bottom the total value he has selected till now from the dropdown.
so eg if pr1 = 5 points
pr2 = 2points
pr3 = 1 point
pr4 = 6 points
pr5 = 8 points
>>>> so on till pr 10

and the person selects from three drop down menus pr1, pr3 and pr4
i want it to show the total to the user immediatly at botom of page?(some kind of summing for the values) in this case it will show 12 points.

and in case it goes above a predefined limit (again taken via db say from vboptions eg 10) it shows a warning.
The products and the values are being taken from db and shown via select boxes.

Any help or code snippet on how to do.
thanks in advance