Hello, I was wanting to know the best way to have a checkbox give a duel value..

example say I have a check box called "comicbook" and I also have a checkbox called "single" and one called "pack"

What i would like is
if "single" is checked then value of comicbook is 5
if "pack" is checked then value of comicbook is 10
if both are checked then comicbook would pass both 5 and 10

any idea's?
thanks