You could always just use a decaptcha website , what they do is provide a api and when the captcha image is shown to your users you then send the image to there server instead of solving it. Then they pay someone in a third world country usually to solve the images for you (about 0.5c) they charge you about 1c per captcha solved so this is how they profit.
Depends on the type of bot it may not be worth it to you , but if you only need 1000 captchas a month it would only be about 10$.
(you need an account to see links)
Is the site I used to use.