I've used PRWeb and was disappointed. I had collected media contacts on my own, and sending the same release to my own list was much more beneficial and resulted in more leads than the one that went out via PRWeb. It costs between $80 - 300 depending on how many contacts you want your release to be sent to. Not worth it in my opinion.
I do recommend PR Newswire, though. It's more expensive but it works. I believe, it's $680 for a release with 400 words. Good luck!