Updating from select

Hi all

I have two related tables:

CREATE TABLE trans_log (id    serial,date    timestamp not null,cost    numeric(12,5) NULL
);

CREATE TABLE products_log (id    serial,trans    integer not null references trans_log(id),cost    numeric(12,5) NOT
NULL
);

So, a transaction can have from zero to a lot of products (1:N)

Ok. I have data in the tables. The transaction tables have their id and 
date, but no costs, so costs = 0.00000 for all transactions.

The products do have their cost set.

I can do a

SELECT trans, SUM(cost) FROM products_log GROUP BY trans;

to get the total amount of each transactions.

What I would like is to update the transactions table with the sum of 
its products cost, some kind of

UPDATE trans_log t SET t.cost =  (SELECT SUM(p.cost) FROM products_log p WHERE p.trans = t.id)

But I'm not able.

Thanks in advance

Thrasher