Additional consideration treated as part of assessable value, not simply added to cum-duty price, clarifying excise valuation. Where the price is not the sole consideration and additional consideration flows from the buyer, the money value of that additional consideration must be treated as an element of the assessable value under Rule 5, not merely added to the declared cum-duty price to work backwards; duty must be computed on the aggregate of the declared assessable value and the additional consideration, and the resulting duty included in the cum-duty price.