Seeking help with a query....

Hi folks, seeking help with a query that I thought was simple, but
apparantly isn't, at least for someone with my knowledge level.

Given a table :

create table atable (
code1 char,
code2 char,
cost int
);

And the rows

code1 code2 cost
-----------------------------
a b 2
d e 4
b a 6
f g 1

I need a ( preferably single ) query that will sum the costs for any
matching pairs of codes regardless of order. That is, row 1 and row 3
concern the same pair of unordered codes (a,b), and the result should show
that the (a,b) pair had a summed cost of 8. I am not able to change any of
the environment or preconditions other than the query itself. I have tried
so many approaches that they aren't even worth listing. Any suggestions
would be very much appreciated.