This may be true for existing fans of the game, but usually when you market a strategy game as having multiplayer, your average prospective player will assume that the game is balanced for competitive play in MP. Fortunately most of those folks stick with Shooters or RTS style games and have little interest in TBS.
Another issue I just though of for E:FE MP would be cooperative play, where the players are on the same team going on adventures, etc (EDIT: I guess I thought of it because Brad mentioned it. ) It would be difficult with the current system to have multiple PC heroes on the same team. Who gets to be the Sov and control the stack on the strategic layer? In that regard how do any of the characters do anything on the Strategic layer? How is order of tactical battle determined? (I know this is planned to be fixed in FE.) There are a lot of bits to consider for MP which do not have to be considered for the SP experience.
There has been a confusion of terms, coop = single player only with human players instead of AI. Rather than meaning actual coop as in multiple players control the same faction. It got refered to as coop as the idea is they will team up against the AI.
One of the biggest oversights in the this discussion is costs involved. In order for the DLC to pay for MP, the cost per person would need to be increased as it will be distributed amoungst fewer people.
Say MP accounts for $10 of the price of the game when it is included with the main game, with a total cost of $60 for the game. It will be priced to that if the expected number of people buy then everything will be paid for with a certain amount of profit. If that is split of into DLC then the cost is divided between fewer people (say 1% of those that get Elemental are expected to want MP, as that is the number they are throwing around). Therefore it is 1/100 people paying for it rather than 100/100, so the cost will need to be multiplied by 100. Thus MP would cost $1000 extra for those that want it, and the base game would cost $50.
Similarly if it accounted for $1 of the base cost, then the DLC would be $100 and the game would be $59. If it is $0.1 then it is $10 and $59.90 respectively. If it is $0.01 then it is $1 and $59.99 repectively. If the cost of MP is still to be covered. This is why the price of DLC seems rediculous, even if it is fairly priced. It is separated from the game to reduce the price as not everyone is expected to get it so it needs to have a greater cost per buyer.
These numbers are assuming the number of people that would use MP translate into buyers for the DLC and that the number remains constant regardless of price. Really the higher the price the fewer people will get it so it would need to be further increased to reach equalibrium... if there would be any buyers at all in which case it is simply undoable. The other option is to decrease the amount spent on MP so that the amount that people will need to pay for it is less; however, it is already something that no one has any interest in as it is.