We use Expert Click for posting releases. While it's not "low cost" it is cost effective if you plan an aggressive PR campaign. For $997 per year you can post 52 press releases. If you were to do that with PRweb.com it would cost you a minimum of $4,160 since they charge $80 minimum per release. For occasional releases PRWeb is a good option though.
We do this, is not only to attract the attention of the media but also to seed the Web with keywords and links back to our sites and blogs. It's one more way people can find us when they are searching for information online.
Here are the three media releases we posted (with links to video):