Depends on where you go. Usually it's anywhere from $3-5 per lb.
Here are some places:
-Crystal Lake - 'Johnny O' $4.50 per lb. You can talk to this guy for hours about nitrous.
-Schaumburg (I forget the shop name) Golf and Roselle $3.00.
-Speedshop in Barrington Road - Dudes SpeedShop or something - for $3.50.
J.E.T.
'97 Red GTP