I just don't see how a spare gold arm could cost more to pack in than a blaster, removable helmet, and alternate set of hands. (Poe) The amount of plastic, tooling, paint etc is certainly comparable.
Of course I realize the answer is so they can sell me the same figure again later, but thats weak.
I see Hasbro trying to maximize their profit on this. And that means multiple versions of C-3PO over the course of several years. Just think of the variations!
-TPM (no coverings)
-AOTC (mis-matched grey/metallic coverings)
-ROTS (shiny gold coverings)
-ANH (burnished gold coverings, silver leg)
-TESB (blasted apart, silver leg)
-ROTJ (silver leg, different midsection)
-TFA (red arm & later gold arm)
And that's not counting any other protocol droid, like TC-14, U-3PO, K-3PO or whatever else. Give Hasbro some time and they're going to get their money's worth out of this mold.