Quite interesting how the authors even mention that this strategy is very economically viable. From the paper: 'Assuming a selling price of $2 per 1,000 solved

Quite interesting how the authors even mention that this strategy is very economically viable. From the paper: 'Assuming a selling price of $2 per 1,000 solved captchas, our token harvesting attack could accrue $104 - $110 daily, per host (i.e., IP address). By leveraging proxy services and running multiple attacks in parallel, this amount could be significantly higher for a single machine.'