Cost of the operation. electric, loan repayments (unless its all paid for) rent etc. 4 months ago BTC was worth 2x what it is now, its a big chunk to lose.
I think it's all paid for... Here's a quote from page 2:
"I figure I spent about $1 million building the existing facility and racking and deployment of the mining hardware, and then we probably spent in the range of $2 million to $3 million on the hardware itself," he said. Rent is another $8,000 a month, power $40,000 a month. "I think we spent probably $3 to $5 million dollars total on the operation. It's paid itself off [in bitcoins] many times over already."