It depends on the advertiser I think. With google ads the advertisers bid and I dont think you get to choose whether its pay per click or pay per view.
The ISP ones are all when product is bought. At one time I used to do ok with those, but over the past few years I've been getting a lot of clawbacks which strongly suggest they've been here, then bought through one of the cash back sites. Now I may get about £20 every few months so I may just remove them and ditch it completely and try doing all PPC or PPV.
The main site gets most of the visits so not much point me moaning here really, but I am going to have to do a serious rethink this year because it is getting increasingly hard when ad revenue isn't even meeting a tiny fraction of my hosting costs. Its not that traffic has gone down, so it has to be ad-blockers.
I think it may be a sign of the times, its increasingly hard to keep smaller independent sites up and running. Whilst I do understand the need for things like the DPA, Cookie Legislation and now GDPR its also becoming a mine field. I am fully compliant with DPA and cookies but GDPR I wish to avoid. It's still a grey area when it comes to forum software, but if they start lumping that in too, then I don't see how a lot of sites will be able to keep going. There's been some discussions over on the SMF software forum about GDPR, but the consensus is that if you're not a subscription based forum then you should be ok.
Not sure yet if it's even been thought through properly when it comes to IP addresses as no-one can seem to make up their minds. In the end it may even turn out like cookie legislation. I spent ages being fully compliant, then when D-day arrived it was suddenly realised that it was impossible for some .gov and EU sites to comply, so they relaxed the rules. There's a lot of people waiting to see if the handling of IP's is also included as personal data and what about email addresses used to verify a sign-up and login. I don't really see how they are going to enforce it.... or else a hell of a lot of blogs and community forums will have to close.