You'd be amazed.
Hell, 100's of thousands of people still respond to email spam to buy Viagra and other drugs, specialty products, etc.
You. Would. Not. Believe. It.
And even so, what are the expenses for a spammer? Whether they send out 10,000,000 or 100 emails, or set a ad on a web site only 100 people go to or 10,000,000, it all costs the same—cheap!
If only 1 in 25,000 buy their product, the advertising cost versus profit is still decidedly in their favor.