well, in the Guitar Hero thread people were doing referrals for other folks without any incentive. I just clicked on a name I knew and boom, he had a referral. It's hard to sweet-talk your way to 7 referrals, though, so some incentive is in order. I personally don't have the cash to offer $20 a head, but I'd still like to nudge people toward using me if they're gonna do it anyway...hence I put up a couple good games I wouldn't mind giving up. If somebody else really does offer $20 apiece, good for him. He knows how to win the game.
Oh, and I've heard mixed reviews on these companies. Some apparently send out prizes quickly, while others never do. Beats me, but what can it hurt if you do cheap or free offers?