Actually, I managed a reasonably large scale free email operation and spammers DO spend money solving captchas.

We had this issue with spammers where after sending 50 emails a day we would ask users to solve a captcha after each email sent. That didn't work.

Upon further investigation and adding some code to track keypresses, we discovered the reason: it had been humans all along, sending spam semi-manually from a cybercafe/sweatshop in Nigeria using an add-on like Roboform as an aid. And yes, these were the usual H3rb4l V14gr4 spammers as well as some Nigerian Princes.

While there is poverty, Captchas are necessarily broken.