Another poster posts...
"Long answer is that what he means by cost prohibitive is the margin requirements. For example at the broker that I use, you have to have $2.50 margin per share to short a stock under $2.50. So if you short 1000 shares of a stock at $2, you have to have $2,500 in your account ($2.50 x 1000). If you want to short 1million shares of a stock at .0001, you have to have $2.5mllion in your account, and 1million shares at .0001 is only $100. So it wouldn't make sense to do. The bright side is that if you do short something down there, you won't get a margin call unless it exceed $2.50, which isn't very likely for a .0001 stock. But again, you'd never short $100 worth of a penny stock if you had $2.5 mil in your account."