And what becomes of the argument that charging rates according to what the traffic will bear, is an ample safeguard against extortion? Many of these small changes in price are diffused in the friction of retail trade;[146] some of them are unfortunately magnified to the consumer, especially under conditions of monopoly. When freight rates on beef go up ten cents per hundredweight, the consumers' price is more likely than not to rise by ten times that amount. But even assuming the final cost to follow the range of transportation charges closely, is it not evident that, so small relatively are many freight charges by comparison with other costs of production, that consumption is not proportionately affected by their movement one way or the other? And yet the entire argument that the value of service principle is a self-governing engine against unreasonable rates, is based upon this assumption. Surely the increased income to the carriers when rates are raised must come from someone. Because it is not felt, is no reason for denying its existence as a tax. But the very fact that it is not felt, undermines the argument that a safeguard against extortion obtains. The theorem that value of service in itself affords a reliable basis for rate making, pre-supposes that freight rates and prices move in unison; a supposition which a moment's consideration shows to be untenable in fact.[147] Such cases must be finally settled by some reference, indefinite though it be, to the cost of conducting that particular service; or rather, as Lorenz puts it, to the extra cost incident to that service. This extra cost may oftentimes be segregated, where the total cost could not be ascertained.[148] That the problem is, however, a most difficult one is evidenced by the periodic controversies over railway mail pay.[149]