The price of STEEM has to drop below 50 cents for that to happen.
You get less than $1 from a Steem Dollar conversion if the final price of Steem after the 3.5 days is below the median price of Steem during the 3.5 days. It doesn't have to drop below a specific amount.
I will be buying up SBD as they go below $1, but I don't think it's a risk-free trade.