well it depends on at what stage your website is, if you are just starting out.. u need to invest a good amount
When I first tried it I set it at $3-$4. It did not result in any traffic so I stopped. I gave it a month, and used a Free $100 coupon so it only cost me a couple of dollars to try it. I Don't know if I will try it again, only had about maybe 2-4 customers.