After some confusion it looks like the pricing for the xbox 360 has been confirmed. It was first leaked on LATimes and later confirmed on ign and GameSpot. LA Times requires registration so use BugMeNot if you don’t have an account.
Xbox 360 core system: $299
Xbox 360: $399
I seriously doubt anyone will buy the cheaper core system considering what you get at the higher price point. According to the price list on GameSpot just adding the harddrive to the core system will add up to the total price of the Xbox 360 $399 package.
Update: Major Nelson has posted the same info but with some additonal pretty pictures of the peripherals and boxes. Nothing really new.