I just found this Captcha that can earn you dollars. It has a simple wordpress plugin to install.

Use the Solve Media CAPTCHA widget on a registration form, comments form, or any Contact-Form-7 v2.0 or newer.
WordPress › Solve Media CAPTCHA WordPress Plugins

Here is the site where to join.

Engagement Advertising Technology | CAPTCHA, Pre-roll, Brand Tags - Solve Media

I am thinking why not, plus it should help cut down on spam even if you don't get rich with it on your site.