Actual DC? Not Virginia or Maryland? 200-300K to start with. More if the entire structure needs steel to keep it's rigidity, which it probably will. Get an engineer in there pronto.

You better have a lot of money to blow in this.

It really, really depends on the condition of the structure and exactly what will be modified. Knocking down walls is cheap. Making sure the rest of the house stays up can get expensive in a hurry. Instead of posting on the internet, you need to a get a structural engineer out to determine exactly what needs to be done and then get some quotes for how much that will cost.