This is a question that many companies ask and most of the time they are happy to just let the advertising agencies do their best in this area.
However, your agency should be negotiating very hard for you, because a small saving on many campaigns can certainly add up.
Of course, most publishers will always offer them an agency discount which was originally based on agencies bringing many paying clients to the door of the publisher. This is not happening quite as much today, but it is still true that agencies can make a significant difference to the revenue streams of publishers by bringing in multiple paying clients.
When you appoint an agency, make sure their expertise is in your marketplace and that they already have a very good rapport with the media in this area. This could make a huge difference to your business.