You're failing to take into account the difference in market size. Take your $2.78 cost per player per civ figure, but suppose that only 10% of the people who buy Civ V will choose to buy the DLC. Then to keep the price per player per civ the same, they'll need to sell it at ten times the price, so $27.80 is the price that "makes sense."
It should be readily apparent that I am looking at it from the point of view of the consumer. The arbitrary ways in which prices are affixed to goods do not concern me, I am merely comparing the value which the individual user gets from the original purchase to that gained from any DLC. To put it simply, it makes very little sense for the consumer to pay 10% the cost of the original game to get the equivalent of, say, 1% of its content (measured in comparative labour-costs, for lack of any other objective measure of value).
Incidentally, if you do insist on looking at the salesperson's point of view, then you must consider the comparative cost in labour as well as the comparative revenue. If the labour necessary to produce a DLC civ is 1/90 of that to produce the game*, then total revenues must approach 1/90 of game revenues to be a similarly good investment. Assuming 10% as many sales as the original game, the price must be 1/90 divided by .10, or 1/9, of it, thus $5.56. This number is of course arbitrary as it depends on an arbitrary variable (we do not know the total time it took to create the civs, compared to all the other mechanics, artwork, etc.), but this formula would be valid given accurate inputs.
*The 1/90 figure equates to 20% of the total game development time originally being spent purely on the individual civs (a reasonably conservative estimate I should think), as it is 1/5 (the total time for all civs) divided by 18 (the total number of all civs), resulting in a time-per-civ of 1/90.
Eh, if you think each civ is worth $5, it's fine. You don't have to buy each one.
There is a difference between subjective
utility and objective
value. Subjectively, it is possible for steel to be more useful than gold (e.g. survival situation in which a metal weapon is needed). Objectively, however, the market value of gold is higher than that of steel, regardless of the metals' owner or his specific metal-related needs.
Subjectively, $5 may or may not be entirely acceptable; this is an individual judgment and it depends upon the individual utility of the good just as much as its general value. Objectively, however, paying $50 for 10 civs compares poorly to paying $50 for 18 civs plus the game within which they are played; this is the simple point I am getting at. This is a judgment of whether an individual action is logical or illogical, not a moral or financial one regarding the company's choice of price.
