Jake, I'm no expert on the PR internal process.. I've personally never seen a company release news without goin through one of these firms.. I've heard that PR Newswire charges a grand per pop.. and Marketwire is a little cheaper on their fee.. I have no proof of this though..
CJ, I think that they just pay whatever firm they want to publish it. Firms that publish PRs after they've been out pretty musch just syndicate from the larger firms to gain publicity.