My question is: is unique visitors the right metric that you should be measuring? On Fiverr.com I can get 2000 to 10,000 unique visitors for $5. http://fiverr.com/gigs/search?query=unique+visitors&x=0&y=0
Can you tie your metrics to something else that might have more value for you, such as purchases, newsletter signups (still easy to fake, but at least takes a little more time), etc?