I like the cleverness and simplicity of the bitcoin mining approach, but the tradeoff between "takes too long, damaging our signup flow" (where anything more th

I like the cleverness and simplicity of the bitcoin mining approach, but the tradeoff between "takes too long, damaging our signup flow" (where anything more than 5 seconds is likely to have a material impact) and "doesn't take long enough, making it too cheap for bots to proceed" may be quite tricky. Charging a buck is extremely simple, and fair. The SA example tickles me. I wonder if the folks who dislike reCAPTCHA would be willing to choose to pay $1 if given the option between the two.