I'm not sure where the the article summary got its notion about the costs. The article doesn't address that- instead it spoke to how much could be made selling the service. From the article:
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.
I think the authors of the article were trying to communicate how much money they could make selling this 'service' to other unsavory agents. It could be a lucrative business given the assumed market rates of $2 per 1k, and the mentioned optimizations could make it even more attractive. It makes me wonder if you could set up the whole thing in a cloud computing environment like AWS and come out ahead.