someone who had been searching for the same answer I'm looking for (there weren't any answers online). The guy went ahead and weighed the wheel and tire combo, and then deducted the weight of the tire (according to the tire manufacturer's specs) and came up with 21 pounds for the fronts. He didn't weigh the rears, but they would obviously be heavier since they're wider.
If the rears aren't much heavier, I'd say the 18 inch 370Z base wheels are a pretty viable choice. They're cast aluminum, so they're not the optimal forged wheels you can get with the 19 inch nismo's. But, they're great looking and seemingly lightweight. They'll fit the car, and you can get a good used set for relatively little money. I came across a site that wants $350 each (front and rear) for a "like new" used set. $1400 isn't bad for a great set of wheels. I bet one could pick up a set with wheels from a 370Z owner looking to change out his wheels for even cheaper than the $1400 plus tires.