If you're worried about the weight of a Brooks, just be sure to go pee before you ride.
Seriously. That's about all the difference a Brooks adds.
(like BSG said, about the weight of a glass of water)
Now, I ride on Brooks B67 saddles, which have springs and a bit more metal to the frame. They are heavier, but I ride so much faster and farther with them that they are WELL worth it!! (the springs are miraculous for adjusting to my lopsided pelvis, I am not giving those puppies up!)
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson