cost variable into function

Hi,

i created a stored procedure (function) under postgreSQL and when i checked deeper into my DB, this is what i get:
....
    END;
$BODY$
  LANGUAGE 'plpgsql' VOLATILE
  COST 100;
ALTER FUNCTION myschema.get_count_languages(character varying) OWNER TO myuser;

to what corresponds the parameter COST 100 ?
it is the first time i see that :-(
thx.
--
Alain
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Windows XP SP3
PostgreSQL 8.2.4 / MS SQL server 2005
Apache 2.2.4
PHP 5.2.4
C# 2005-2008