It is amusing how people assume things and stick to their assumption for probably ever without knowing they are on wrong premises.
A fellow passenger in the train asked me about how Google AdSense works. His expression of disbelief was genuine when he realized that the money through AdSense is totally free; you don't have to sell anything to make money. This concept has captivated millions in the world.
Ivory Products maker:
The fellow passenger is in ivory goods making business. He makes a wide range of ivory products that decorate the showcases in the homes of presidents of nations to bloggers like you and I.
His assumption was if ivory is expensive, then, the keywords related to ivory should be very competitive among advertisers of ivory product dealers and publishers should be making big money on ivory related blog posts and articles.
Ivory pays less:
Actually, ivory pays less; it is around $0.94 per click.
Therefore, his assumption is wrong and if I assume that ivory is cheap because of its low cost bidding by Adwords managers, I am also equally wrong.