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@pobox.com