Calculated fileds in pg

Is it possible to set up a field during table creation to be a
"calculated" field, i.e. the value is calculated from other values.

For example:

Table A:

id   integer primary key,
B_id integer references B(id),
cost integer default 0

Table B:

id    integer primary key,
total integer -- select sum(cost) from A where B_id=id;

If not should I be looking at implementing this with a view or maybe a
trigger?

Thanks,

Jc