How much to charge for an web ad?

Hi All,

For one of my clients i am designing an ad system for them to use on their website.
However they have also asked for information regarding how much they should charge their ads to be displayed on their website. At this stage they have not decided whether or not the use gifs or text, but i do have some stats for the usage of their website, 101214 hits last month so about 3000-5000 hits a day, and a total of 6199 sessions for last month, page views 7993 for last month.
So if anyone has any suggestions on a pricing scheme for this, whether x amount for a week, month, quarter etc let me know.

Thanks for ya help.
Michael.