You and many others always refer to Interactive Brokers when talking about shorting penny stocks - here are the Margin requirements for IB.
Maintenance Margin
30% * Stock Value if Stock Value > $16.67 $5.00 if Stock Value < $16.67 and > $5.00 100% * Stock Value if Stock Value < $5.00 $2.50 if Stock Value <= $2.50
"For short positions, the maintenance requirement on stock greater than $5, is $5 per share or 30% of the stock value, whichever is greater. For short positions in stock where the last sale price/share is less than $5, then the maintenance margin requirement is $2.50 per share or 100% of the stock value, whichever is greater."