If these were typical desktop computers, the CPU working at 100% can use some 50 W more than the one staying idle. 5000 computers at 100 % CPU produce then the load of 250 KW, resulting in 6 MWh per day which at 10 cents per KWh costs 600 USD per day, or 220000 USD per year. Finally, 9 years that he did it make almost 2 million dollars (!) for electricity that he managed to spend.