Well for the affiliates it's CPA and leads, however for the companies issuing the loans it is on the fees they make the majority of the money, essentially the fees are hidden intresst rates but in legal terms it is two different things at least in most cuntrys which is what the loaning companies takes full advantage of, most consumers only think about the intrest rate and either doesn't understand the financial "mumbojumbo" used to disguise the actual cost or simply doesn't pay attention to it.
I'll give you an example that I just recently saw:
Loan: $1000
intrest rate 3%
administration fee: 23,5%
fixed rate fee:15%
(fixed rate meaning that the 3% intrest rate is secured and not subject to change regardless of the ups and downs of the financial market)
Now imagine the ad looking something like this: Would you like a little extra cash for the month GET $1000 @ ONLY 3% intrest rate. administration fee: 23,5%
fixed rate fee:15%
Now ofcourse the ad copy was better than what I just did from the top of my head and it was a tv comercial, where the admin fee and fixed rate fee was only displayed for 20 seconds, but that is enough for them to claim that the consumer is able to make an edjucated deccission in the matter plus it is all also written in the terms to which the consumer agrees but nobody really reads.
Last I heard in the UK the whole first page for payday loans in now manually checked by Google. I don't know the status in other countries though. That could be worth pursuing....when it was big in the UK though the front page was almost pure blackhat churn and burn and I heard a day on page 1 was worth about £10K in aff revenue. This is far from first hand info however so anyone who has actually worked this niche please correct me if I'm wrong.