Transfers from/to exchanges cost ZERO fees, gotta love Steem. So yes, you could transfer that 0.5 SBD on bittrex, sell it, buy STEEM, and transfer that back. However, there are commission fees for each transaction on the exchange itself.

In the end, the internal exchange remains the best and quickest solution. I recently bought 60 STEEM with 10 SBD on the internal.

10 SBD x $12 = $120
$120 / $2 per STEEM = 60 STEEM

The prices are approximate when I did the transaction. But you get the idea.