How to sum monetary variables

I'm building a table (which is a report that has to be printed) with a bunch of items (up to 300 in some cases) that have unitary price (stored in a numeric(9,2) field), how many there are, and the total price for each item. At the end of the table there is a total of all the items. The app is running on PHP and PostgreSQL is the backend. The question is, how do I get the total of everything? Running it on PHP gives one value, doing a sum() on the backend gives another, and I'm starting to notice that even using python as a calculator gives me errors (big ones). Right now I'm doing the maths by hand to find out who has the biggest error, or if any is 100% accurate. Any ideas? -- Martín Marqués select 'martin.marques' || '@' || 'gmail.com' DBA, Programador, Administrador