If it is a continuous project with your client, Then charge what you would like for an hourly rate... I.E If your ads take you around 20-30 minutes to complete and your wanting $30 an hour, Charge your client $15 per Ad you produce for them, If its a one time payment just try come to a decent agreement with your client, Try ask out of the box questions on how much he has paid or was willing to pay for ads before.
Good luck.