1 lb of powder would be 16 oz (oz is a measurement of weight, not volume)
As far as value, it's running around $20/ lb, but that can vary up or down by $5 depending on the type.
Now if the can is opened, that drastically reduces value... once the seal has been broken on a can, many won't touch it second-hand unless it's DEEPLY discounted. There have been instances where faster burn rate powders have been placed in slower burn rate labeled cans... which would result in detonating your firearm like small bomb. Personally, I wouldn't buy an opened can of powder unless I knew the seller personally and he vouched that it hadn't been mixed or altered.