In determining value, however, as regards sales from one to another of affiliated companies or persons, or under other circumstances where the relation between the buyer and seller is such that the gross proceeds from the sale are not indicative of the true value of the subject matter of the sale, the Collector shall prescribe uniform and equitable rules for determining the value upon which such privilege tax shall be levied, corresponding as nearly as possible to the gross proceeds from the sale of similar products of like quality or character where no common interest exists between the buyer and seller but the circumstances and conditions are otherwise similar.