We currently get about 1327 hits a month World Wide. Each ad is displayed randomly. The number of impressions and prices vary by country, category and size.
Ads can be placed by Country, Category or World Wide. Category Ads take preference over Country Wide ads and Country Wide ads take preference over World Wide Ads.
ie: If a Category Ad is sold for Houses it will be displayed before a Country Wide ad, and a Country Wide Ad before a World Wide Ad.
So it's best the find a sub category that best suites your product.