Satisfactory recognition rate
The Yahoo CAPTCHA is broken press-release reveals some numbers. I’m highlighting them:
It’s not necessary to achieve high degree of accuracy when designing automated recognition software. The accuracy of 15% is enough when attacker is able to run 100 000 tries per day, taking into the consideration the price of not automated recognition — one cent per one CAPTCHA.
So:
* 15% recognition rate is enough
* 1 cent per 1 CAPTCHA when using monkeys