The short answer: a quick search tells me that a Shimano 105 front hub is about 200 grams and an Alfine dynamo is nearly 700 grams. So, roughly 500 grams, or about 1.1 pounds.

However, you might be able to minimize the weight penalty if your current wheel is pretty chunky. Building from scratch, you can go with a decently light rim and spoke combo, making the overall difference a little less.