Based on freely available specifications (http://www.spaceandtech.com/spacedata/elvs/atlas5_specs.shtml) for the Atlas V-551 launch vehicle, the density of aluminum you stated and a best case estimation of the payload weight (just over 18,000 lbs based on the above site), I calculated $6084/lb, which comes out to $1789 to launch these little guys.
HOWEVER, with the dimensions of a minifig from here (http://snively.blogspot.com/2009/04/lego-minifig-dimensions.html) and here (http://snively.blogspot.com/2009/04/lego-minifig-dimensions.html) the volume of a minifig is closer to 0.59 in^3 so the total works out to just about $1054.
And by the way, you had a typo in your result up above, though your answer is correct.