Using new.id in the WHERE condition of one UPDATE inside ON INSERT rule

I would like to avoid using "SELECT MAX(id) FROM my_table" (it doesn't seem a
good method), but if I use "new.id", I get new id values, not the "id"
inserted with the rule, and the condition is always false.

Complete example (it works because it doesn't use new.id inside condition):

DROP TABLE IF EXISTS my_table;
DROP TABLE IF EXISTS my_other_table;

CREATE TABLE my_other_table (
id serial PRIMARY KEY,
my_other_cost INTEGER
);

INSERT INTO my_other_table(my_other_cost) VALUES(155);
INSERT INTO my_other_table(my_other_cost) VALUES(277);

CREATE TABLE my_table (
id serial PRIMARY KEY,
id_other INTEGER,
my_cost INTEGER
);

CREATE OR REPLACE RULE my_insert AS
ON INSERT TO my_table
DO ALSO
UPDATE my_table SET my_cost = my_other_table.my_other_cost
FROM my_other_table
WHERE new.id_other = my_other_table.id
AND my_table.id = (SELECT MAX(id) FROM my_table); -- I want " = new.id"
here, but doesn't work as I expect

INSERT INTO my_table(id_other) VALUES(1);
INSERT INTO my_table(id_other) VALUES(2);

SELECT * FROM my_table;

Thanks
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