Trigger cant find function

I seem to be having difficulty creating a trigger. I have creted the
function and tested it which seems to work fine: -

CREATE FUNCTION get_prod_cost_price (varchar, int8) RETURNS float AS '
DECLARE
cost FLOAT;
BEGIN
SELECT cost_price INTO cost FROM product WHERE code = $1;

IF FOUND THEN
UPDATE order_detail SET cost_price = cost WHERE order_detail_id=$2;
RETURN cost;
END IF;
RETURN 0;
END;
' LANGUAGE 'plpgsql';

dvd=> select get_prod_cost_price ('DVD368', 10027);
get_prod_cost_price
-------------------
9.81
(1 row)

Now I try and create the approprate trigger and I get the following:-

CREATE TRIGGER tg_update_order_detail AFTER insert
ON order_detail FOR EACH ROW
EXECUTE PROCEDURE get_prod_cost_price('product_id', 'order_detail_id');

ERROR: CreateTrigger: function get_prod_cost_price() does not exist

It is clear that it does exist so why does the trigger creation code not
find it?

Thanks in advance for any pointers.

Graham