Banner ads are sold on a CPM* basis. The CPM varies by publisher, bur ranges from $10-$50 per thousand.
There is not a set amount you must spend per month, but a typical ad budget will range between $1,500 - $5,000 per month depending on the sponsors goals.
An example campaign budget of $3,000 running on a publisher with a CPM of $10.00 would mean the sponsor would receive 300,000 viewable impressions during the campaign.
*Cost per 1,000 viewable impressions (CPM) - is a pricing method used to denote the price of 1,000 viewable impressions tracked. An ad is counted as "viewable" when 50 percent of the ad shows on screen for one second or longer for display ads and two seconds or longer for video ads.