Hi Richard,
It depends on the content of your page, because google adsense read the text and then generate a relavent ad for those who are interested in you text. Got it?,
Accordingly the ad could be a cheap one or a highly competitve based one. (i.e. not all of the companies are paying well to google for their ads).
If your text are talking about a medicine and a very high competition is rolling around between the advertisors then google ads for this kind of product will be costly due to price bidding scheme of google adsense.
I think the above is enough to understand the situation of ad sense.
if you need anyfurther question that feel free to ask.
Yours
Flashback
www.fbservice.ws - people are making thousands worldwide (did you miss the opportunity).