To be less flippant: it's probably a bit of a gamble with small budgets. You may or may not get referrals out of a small campaign. The larger the budget, the more likely you are to get referrals, making it more of an investment.
And on that note, I should add that the thing with referrals is the more you get the better. It's about having a whole bunch of tiny little streams combining to make a bigger one. You may get lucky and get a really prolific referral, but by and large it's the many ordinary referrals that will steadily contribute to your own earnings.
That's true with every site, not just Cointiply.
In Cointiply's case, you can advertise it without ticking any of the boxes (my ad was approved which is evidence of that).