Retainer, per lead, or a cut of closed deals: which one actually aligns anybody

Working out how to pay for lead gen when I start buying, and every structure I look at breaks in a different place.

Flat retainer plus ad spend is the standard. The agency gets paid whether or not the campaign works, which sounds bad until you consider that they also have no reason to stuff the pipeline with junk to hit a count. It funds the boring maintenance work, the negative keyword lists and the weekly location reports, that nobody pays extra for.

Per qualified lead sounds like the fair version. The problem is the word qualified, which someone has to define and then argue about every month. And it pushes the agency toward volume, because their revenue is a count. I have watched a friend spend more time disputing lead quality than working leads.

Per contract or a share of assignment fees puts the agency on the same side as me, in theory. In practice they cannot control my follow up, my offer, or how fast I answer the phone, so they are being paid on my execution. Good shops refuse it for exactly that reason, which means the ones who accept it may be the ones who cannot get retainer clients.

My guess is the answer depends on how much volume you can absorb and how good your own follow up already is, but I would rather hear from people who have run more than one of these.