But what is a fair price? That depends on the traffic, but we are positing that detection of "is this a real user" is not possible, right?
Traditionally, advertisers have gone by Nielsen style ratings for broadcast media (pay people to track what they consume, extrapolate) and circulation numbers for print media. In our hypothetical world the former would still be possible but the latter wouldn't. Unfortunately, in addition to being really inefficient, if you pay people to track what they consume you will essentially never compensate niche publications. This strongly promotes centralization.
Privacy Pass / Trust Tokens / etc seem much more promising to me?
Couldn't the price just be based on the actual payoff the advertiser gets (aka increased product sales)? The publisher is incentivized to set the maximum price that the advertiser will pay, and the advertiser is incentivized to get the most bang for their buck, so at the very least they would never pay more than what the ad brings them in terms of revenue.
Over time, this should reach an equilibrium. Niche publications may have to charge low prices at the start as they build their reputation among advertisers, but I think that's a worthwhile price to pay if it means better privacy and eliminating a problematic advertising model of CPM/CPC (where fraud is possible and tracking is required to battle it).
Getting the initial price is going to be hard, but over time, rates will start to become known.