Breezer, I think you need to look at several factors here.
1): What your advertising
2): Whether the target audience is UK, Euro or World wide.
3): The type of site you intend to advertise on, will it be as per no2 above.
4): Whether the funding will be personal or corporate.
5): Whether the ad will be an animated Gif with a link, a static advert with a link or even a flash advert with a link. (this is because a lot of companies have different rates for differnt types of ad dependent on the bandwidth it will use, and a few arbitary factors they seem to pick at random!)
It is not that easy to be honest.
Sometimes the easiest way to mutually advertise so that it costs neither party a penny.