For an E36 (probably not including the 318ti) it's as follows:
Standard OEM hood: 45 lbs
Carbon Fiber hood (on average): 12 lbs
You save: 33 lbs
Yeah, it's a marked weight difference, but I still don't think it'll make that much difference in a car that weighs north of 3,200 lbs.
Plus I think they look goofy as hell on non-black cars.