The two models you've named are the real ones, and the market has settled into a rough compromise between them. Pay on close aligns incentives and puts all timing risk on you. Pay per lead gives you predictable revenue and puts quality risk on the buyer. What operators actually run is a small per-lead fee that covers your data and labor cost, plus a bonus on close. The retainer version you're describing is the same idea with the per-lead piece bundled into a monthly number.
For that to work you have to define a qualified lead so tightly that nobody argues. Verified owner name from public records, a working phone number, confirmed occupancy status, a photograph, and a note that the owner has been contacted and expressed some willingness to discuss selling. If "qualified" is vague, your retainer becomes a monthly fight.
One cost people underestimate: data and skip tracing aren't free. List services and skip trace credits commonly run from a few cents to a couple dollars per record depending on volume and provider, and mailing or texting on top of that. Price your retainer above your true cost per delivered lead, not your cost per record pulled, because most records don't become leads.
A retainer relationship also changes how your role reads. Charging money on a schedule, with defined deliverables, starts to look like a marketing service, which is fine. What matters is that you keep passing information and never step into negotiating terms for either side, and where that line sits is set by your state's licensing statute, so have an attorney in your state look at your agreement before you sign six of them.
