Nitrous prices vary greatly -- when I was flying them, it could be anywhere from $3 to $6 per pound.
Where you purchase it, and how, makes a great deal of difference. I originally was filling a 20lb bottle, which meant it cost me about $4.50 per pound.
Then, our club started flying more, so a friend leased a K bottle. He paid about $3/pound, but had to factor in annual bottle lease, so how much we used in a year affected the cost per pound quite a bit. If we only used one bottle a year, it stayed at about $4.50/pound. If we used more, it would've dropped.
-Kevin