sorry to say this, but 20 referrals would not generate enough traffic on the site to justify a D90's cost. This is assuming this site is legit, and even with 50% people do not get the referrals but tries anyway, the site traffic would still be rubbish in terms of unique click/visit and it won't have any significance for advertising purposes, which most of the commercial sites do either by selling ad space on the site or by cashing in small (i mean pennies) commissions via each unique clicks.
this is definitely a scam or hoax.