I am actually using one such micropayment system. This is the recaptcha solver that solves every recaptcha I encounter on any web page, and I pay 3 cents for th

I am actually using one such micropayment system. This is the recaptcha solver that solves every recaptcha I encounter on any web page, and I pay 3 cents for that service. I refill my account with $5 once every few months. No decision is involved, in many cases I overpay, but no complains here, still worth it. So if a mechanism exists that allows to automate the decision "pay or not to pay", even if the mechanism is not perfect, it could work.