Rite Rant: I am sick of people complaining about reviewers
If you send out one PDF review copy to a blog and you get one purchase from that review you have covered your cost of giving away a copy (this is assuming the blogger would have bought a copy of your product).
If you get 2 purchases then you have seen an increase above your potential sales numbers. You also might not see a sale that day, it may go into the cart or into a wishlist for a later purchase.
Lets take one day of DrivethruRpg's featured reviewers yesterday: there were 11 new products released, yesterday was a slow day, that means one single reviewer has 330 products a month to choose from. If you don't beleive me go look at Megan Robertson's List of reviews, she is a workhorse and featured reviewer, and no one pays her to review products. If I could pay her I would and that's another reason I started my own magazine so I could pay people like her to do reviews of products (and not just mine).
I am so sick of people complaining about OBS featured reviewers, I have been doing this for nearly 4 years, release a PDF every week and its still difficult to get a review, UNLESS I WORK FOR IT. I talk to the reviewers, I contact them via email, I privately discuss their review and what I can do to improve my product, I visit their blog and comment on things besides my products. I send them advance copies of special works before other reviewers have access to them. I also actively ask for a review.
I even ask for a review of products via my social network contacts.