I personally use bittrex (this is also what the backend of this site uses) and I'm not sure where you take your 50% but the current steem/btc exchange rate right now is : 1 steem = 0.00459969 BTC (1).

Keep in mind that this value changes all the time so your steem might be worth more or less BTC depending on when you exchange it. And of course, using an exchange means you have to pay a transaction fee but those are normally very low (<1% (2)).