It is just a price with which a certain cargo is delivered from the place of origin to the place of destination.It works on the principle of “charge what  trade can bear” . The rates are made even high until shippers begin to look for cheap rates some other place.

It depends upon :
a) the nature of cargo
b) the path for transportation [ like ship or truck ]
c) weight of cargo  and
d) distance to its destination.

Some shipping services uses dimensional weight for calculations which helps them in knowing both weight and volume of cargo at the same time.