There is a relay that requires a pubkey to have a PoW minimum of N - I forget the threshold and url.

It’s difficult as I think PoW to $ calculations are hard. And then server ROI vs deplete phone battery ROI to generate a PoW gives asymmetry.

Which is why I liked PoW aggregation - sum of your PoW over time. But a spammer persistent enough can also build free PoW due to randomness or they can spend.

It may end up more of an indicator rather than a single value truth test.