Digital advertising has a cost line it hasn’t known how to talk about for the past year: the bill for AI infrastructure. According to a Digiday report dated July 27, 2026, agency holdcos are resolving it through an implicit trade — they absorb the entire AI infrastructure cost, in exchange asking the client to route a predetermined share of the media budget through “principal inventory.” Principal inventory is stock the holdco buys wholesale and resells, bundled with targeting and placement services, at a markup — one of the industry’s most profitable lines. In one offer made to a CMO, that share reached 70%; some negotiations opened at zero fees and settled somewhere in the middle.
The reading from TAU founder and former WPP executive Robert Webster is blunt: “Agencies like to claim they have invested a lot but much of it is manufactured to justify exactly this — skimming money out of media.” Ad executives speaking to Digiday concede a more measured truth: nobody, including the holdcos, actually knows what a fair price for a token looks like yet. Building a transparent billing model takes time the market doesn’t have; the client wants a number today, not once the industry works something cleaner out. Routing AI cost through principal media is what happens when a real cost urgently needs a home, and the only infrastructure built to absorb pricing risk at scale is the one already sitting there.