First make sure of what is actually being counted
Overlapping bodies can cause the same area to be counted more than once. Before calculating weight, make sure the metal parts form a correct volume, with no duplicates or overlaps that distort the result.
Volume is only half of the equation
The same volume weighs differently depending on the metal or alloy. Once the volume is right, the metal type lets you translate it into a weight estimate.
CAD weight is not necessarily the weight of the finished piece
Casting is followed by cleaning, bench work, polishing and setting, and these processes can change the weight. When a target weight is defined, it is important to clarify whether it refers to the casting stage or to the finished piece.
The calculation needs a sanity check
Even when the software shows a precise number, a professional should ask whether it makes sense for the size, structure and type of piece. Numerical precision is no substitute for a professional sanity check.