I looked at the web site that makes those seats
and it showed the weight but I dont recall what
it was.. it wasnt bad at all... but if you think
of it this way.. a light seat doesnt have a lot
of cushion to it but a heavier one MIGHT have more
but also the material that is used now days tends
to be lighter weight and they tend to use foam as
the item to give it shape(which is light)
EDIT
the seats are made by Pro car/Scat and weight
ranges from 25#-30#