The mining process requires energy consumption and human resources for the maintenance. In order to cover these fees all users are issued with maintenance and electricity fees (MEF) accordingly to their contract type and hashrate. 

The MEF is linear and equals:

SHA-256 (Bitcoin, BitcoinCash) - 0.0035 USD per every 10 GH/s per day.

Scrypt (Dogecoin, Litecoin) - 0.005 USD per every 1 MH/s per day.

ETHASH (Ethereum) - 0.004 USD per every 1 MH/s per day.

X11 (Dash) - contracts are not subject to any fees.

Maintenance and electricity fees are not billed in an invoice. Instead the fees are deducted from the customer's balance at the moment of daily payouts. Despite the fact that balance is accounted for in cryptocurrency and the fees - in USD, the fees are deducted in cryptocurrency according to the current exchange rate which is beneficial for the customer in case the exchange rate trend is positive - more cryptocurrency remains in the customer's balance.