Diffcult query

Hi folks. This is giving me fits. I feel like it oughta be simple, but
apparantly its not...I think.

Given a table :

create table tablea ( code1 varchar(32), code2 varchar(32), cost    int
);

and the rows

code1    code2    cost
----------------------------------
"aaa"        "bbb"    2
"ddd"        "eee"    3
"bbb"        "aaa"    6
"ggg"         "hhh"    4

I need a ( preferably single ) query that will sum the costs for all unique
pairs of codes, without regard to column order. That is, for summing
purposes, rows 1 and 3 are identical, and should produce the sum of 8 for
the unordered pair("aaa","bbb"). It should also, of course, prevent the case
where ("bbb","aaa") is considered a seperate pair. Any ideas would be much
appreciated.