Well, there are other costs to creating DLC which may explain why they cost more per content than a game. For instance, the added bandwidth for downloads needs to be paid for, and servers must be maintained for the DLC content. There is the cost of producing the content itself, which is probably similar to the cost of the content that ships on disc on a per part basis.
But there is also another cost, which is actually quite expensive. Opportunity cost. Opportunity cost describes possibilities that are missed in pursuing one choice over another. For instance, by going to college, I'm missing out on income of several tens of thousands of dollars that I could have made by going straight into a full time job. So, with respect to DLC, the opportunity cost involved here is that Polyphony would be spending man hours and other resources working on adding additional content to a game that has already been released, rather than spending these resources on their next project. This is why DLC tends to cost more per unit, than the content already included in a game.
As for whether the game could be considered complete or not, I could be wrong (naturally), but Polyphony doesn't strike me as a company that would release an unfinished game and then make people pay for content that they purposefully left out to use as DLC. That doesn't make much business sense either, because that would cause them to lose sells to competitors (not just racing games, but games in general). After all, given a choice between a complete game and an incomplete game for the same price, most people would chose the complete game, unless there is something special to them about the other game that they feel is more valuable.
). I think that they will put everything they have into the retail game then maybe work on DLC later which will probably take them years!
