0.05 cents even. The beautiful thing is, minuscule amounts of micropayments are already enough to fix the incentives
The status quo with those js PoW pages doesn't really benefit the server owner at all, it's wasted energy.
I mean proof of work is always wasted energy, but I figure it's better to kill two birds with one stone.
CoinHive was one example of this. (I think this is a correct link? https://github.com/cazala/coin-hive). Although I think ideally you would want to have some sort of browser plugin or app that runs on bare metal instead of a proof-of-work in the browser, because RandomX is designed such that it's slow when implemented in JS (https://github.com/tevador/RandomX/blob/master/doc/design.md)