the service costs 5 mbtc per use. as of this writing (2016-12-24), each use of the service creates 2 transaction outputs. one of them holds the identifier 0x444f4350524f4f46 (which is 'docproof' when converted to utf-8 / ascii) with the sha256sum of the document whose existence at the time of admission of the transaction into the blockchain is proven appended. this transaction output is provably unspendable because it's marked as such via an op_return at the beginning of the output script and doesn't hold any value. the other transaction output holds 4.9 mbtc and pays the service's operators. 0.1 mbtc is paid as a fee to the miner admitting the transaction into the blockchain.