Above-ground walls are typically cost-effective to insulate, for a number of reasons:
they are more exposed to wind than foundation walls are
they are less solidly built than foundation walls
until the 1960s, they were usually built without any insulation in them.
Insulating above-ground walls usually involves drilling holes either on the inside or the outside of the wall and blowing cellulose insulation into each wall cavity. Since the cavities are usually only 16" wide and it takes two holes to fill each one, that's a lot of holes. If the holes are outside, they are covered with round plastic caps; when a house already has vinyl siding, these caps may be concealed behind the siding. If the holes are drilled inside, they are covered by a chair rail and crown molding.
