Really depends upon what you are selling and who your target audience is.
If you are selling/promoting a product, website, or service to whom are you selling/promoting it to? Who generally buys it? What age group? These are all things you need to consider before spending a dime in advertising.
Are you marketing your product to men or women? Americans or Australians? To kids, adults, or senior citizens?
Obviously if you are marketing a website that caters to teenagers you don’t want to advertise on a site that sells wheelchairs and walkers…no matter how great the price is to advertise there.
If you’re marketing your web design services you want to advertise on sites that cater to the 20-45 crowd.
If you’re marketing software you want to advertise on computer-based sites that cater to tech-minded people.
Don’t throw money into a wasted effort…think, before you buy.