It's not a cost. They spend electricity and get money back. It's basically an indirect currency exchange. They obviously do it because the resulting conversion rate is slightly better than the market rate.
No doubt people get "money back". Contrast this however with a system where "numbers" (= the new "Randomcurrency") were randomly, or evenly distributed among the general population, which could be done basically for free.
Compared to that, there is a cost, if you will. n powerplants had to be built, or x tons coal had to be burnt to do all the calculations. Does that make sense?