How much does the NXT brick weigh?
Without batteries installed, the NXT brick weighs 159.3 grams, so that's about 0.351 pounds. 

Do take into account the added weight of what ever batteries you have installed, though, as that will almost double the weight, and not all types of AA batteries weigh the same. If you're using a rechargeable battery, then for reference, the original NXT rechargeable battery weighs 155 grams (~0.341 pounds), and the newer rechargeable battery weighs 136 grams (~0.2998 pounds).

The weight of a single AA battery according to www.allaboutbatteries.com are:

Alkaline: 24g

Carbon-Zinc: 19g

NiCad: 29g

NiMH: 26g

Lithium Ion: 25g