[lxgr]

I could see a future in which ad-funded web sites charge agents some tiny amount per request instead of outright blocking non-human visitors.

Besides, you're already effectively giving agents money (tokens aren't free, and agents decide when to stop consuming new ones), this is just more explicit :)

[ses1984]

Why not have sub agents talk to a payment service? Sub agents should ultimately be associated with a human or business entity, the human or business entity could provide a single payment service for agents.