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4#'s roughly....now once you add some sauce and some of the drippings back to the meat, it will bump up a little. You are cooking water and fat out of the meat and off the surface.
As others have said, it is hard to get exact. If I am trying to calculate portions I figure 40% loss to be on the safe side. I also throw out every piece of fat I see while pulling and I know that other people will leave some fat so your yield can vary depending on your preference.
All meat looses weight once cooked. That's why a 1/4lb burger at any fast food place doesn't way a 1/4lb once cooked. Butts are on the high side of weight loss due to all the fat.