Well, far from being definitive, but it looks like a regular pc will cost around $227.52 (350 watt psu, $18.96 per month) in electricity per year to run 24/7.[1]
(calculated using $0.06 per kWh, which is rather low for most people in the US)
So, 5,000 machines running 24/7/365 would cost around $1,137,600 annually (5,000 machines * $227.52 per year) in electricity usage alone.
Of that 24 hours per day, we can assume 8 hours of each day the district expected to pay for them to run, so that leaves ~16 hours the district expected the machines to be asleep or off.
24 - 8 = 16 (about 66% of the 24 hour period)
So, the admin was consuming about $12.51 per machine per month the district did not expect.
This comes out to around $750,600 the sys admin potentially cost the district, for a single year. He ran this scheme for 9 years -- so...
$750,600 per year * 9 years = $6,755,400 in additional electricity costs
Keep in mind this is likely rather low, since most places in the US have a significantly higher per kWh fee than the $0.06 per kWh used in these calculations.
I'm certain that this sum alone was enough cause to terminate the sys admin.
Think about all the things the school district could have spent an extra ~$7MM+ on...
[1] http://www.dslreports.com/faq/2404