I just took one two-layer cel that I just got
(thanks Vamppy!) downstairs to put on a letter
scale.  Including the cel bag, it weighs one ounce
(25 gm).  So it would take 32 cels at this rate to
weigh one pound.  Of course, oversized cels would
weigh more, as would cels with a lot of cel paint
on them.  But probably your portfolio just
seems like it weighs 20 lb.