Deter unwanted click-throughs
This might also seem rather odd, but as much you want users to click through to your site, you don’t want them if they’re not the right sort of users. By including a deterrent within your ad—such as a price or another barrier to entry—you are encouraging those who can afford your product/service and deterring those who can’t.
Click-throughs are only of value to you if those users then do something on your site—be it register, contact you or purchase. Everyone else is no more than a casual visitor, and too many of them will cost you money with no ROI.
For example, Espressio’s flagship product is their extra-large hardback photo book priced at £17.99. For some users, that’s too high—and therefore Espressio doesn’t ...