It depends on where you buy it.
My local gun store, and it's the only one in the area that sells it, sells it for $26 per pound. You can buy it from Back Creek Gun Shop in Winchester, VA, for about $17.50 a pound. You can buy it from Powder Inc for as low as $17.10 per pound in 25 pound lots. $26 per pound in 5 pound lots. $20 per pound in 10 pound lots. The Powder Inc prices include shipping and hazmat. The Back Creek prices do not.
http://powderinc.com/catalog/order.htm
http://www.blackpowderva.com/
So, basically $17-$26 per pound.
Steve