Google just released Pay Per Action, a major addition to their previous "Pay per click" model. 

PPA could have some significant, and positive applications for us at BigOven.com, Escapia.com and AdventureCentral.com, much more so than pay-per-click (PPC).  Though I’ve not yet tried it, in theory, this finally lets us bid much closer to our marginal profit for various actions, rather than just a lesser sum for a substantially lower-qualified audience. 

Queue, stage-right: precision.  Finally.  More on this later.