The fact is that there are some minimum values of the sums of money that when investing in advertising can give a conversion. The main thing is to guess these values and make the right decision - if money is not enough, then do not start a campaign that will be known to be losing.
Taking into account the fact that everything can not be foreseen and forecasted, advertising is turning into some kind of casino. It is better to pay partners for specific clients than for "probabilities."