From the study:

"In terms of cost, we estimate that – during over 13 years of its deployment – 819 million hours of human time has been spent on reCAPTCHA, which corresponds to at least $6.1 billion USD in wages. Traffic resulting from reCAPTCHA consumed 134 Petabytes of bandwidth, which translates into about 7.5 million kWhs of energy, corresponding to 7.5 million pounds of CO2. In addition, Google has potentially profited $888 billion USD from cookies and $8.75-32.3 billion USD per each sale of their total labeled data set."