As I understand it now, it doesn't really change the cost of "The Avengers much at all. They were always obligated to pay Paramount 8% of the gross for distribution fees (Which averaged $56m per Marvel film apparently). Additionally they paid back all of the distribution and marketing expenses that Paramount took on.

Now, Disney will pay that $56m to Paramount up front, but do all of the marketing and distribution themselves (which, just as it would have been for Paramount, will be taken out of the gross.) Assuming the Avengers and Iron Man 3 make at least as much as the Marvel average, then it comes out to equal costs. (It does mean, however, that Disney has to set aside the cash in their own budget now for the marketing campaign, and they need to devote the time of their own marketing people.)

On the plus side, for the same cost, Disney gets to make all of the decisions and not have to consult with Paramount at all. Paramount is guaranteed to make their normal share of profit off of the films and doesn't have to do any work at all. So... a pretty sweet deal.