Calculating default values on insert?

I'm trying to calculate the default values of a column when I insert a
row, based on the values of other columns. It's something I thought
should be simple enough, but I can't seem to figure out how to do it.
Here's a basic example of what I'm trying to do:

CREATE TABLE money (
amount numeric NOT NULL,
currency text NOT NULL DEFAULT 'USD',
currency_per_usd numeric NOT NULL DEFAULT 1.00,
usd_amount NOT NULL DEFAULT (amount / currency_per_usd) -- This fails.
);

INSERT INTO money (amount) VALUES (50.00); -- 50 U.S. dollars

INSERT INTO money (amount, currency, currency_per_usd)
VALUES (
50.00,
'EUR',
1.25
); -- 50 Euros at an exchange rate of 1.25 Euros per U.S. dollar

SELECT amount, usd_amount FROM money;

amount | usd_amount
--------+------------
50.00 | 50.00
50.00 | 40.00

I realize that I could calculate the usd_amount when I do a SELECT:

SELECT amount, (amount / currency_per_usd) AS usd_amount FROM money;

But it would be nice to be able to calculate the default value when I
INSERT the values.

Is there a way of doing this, or should I just create a VIEW that will
do the calculation for me?

--
Robin Munn
rmunn(at)pobox(dot)com