Insulation is relatively cheap, certainly when compared to the possible gain in terms of heating costs. Rolled insulation or "batts" of insulation can be put up very easily if your walls are open. If your walls are closed foam is still a possibility.
The only reason that I have heard of for insulating interior walls was for sound deadening efforts. There is no heat gain or loss advantage to insulating interior walls.
A lot
One can prevent damp walls by applying a sealant to the interior walls and floors of rooms in the space. One can also prevent damp walls by insulating windows and doors against humidity.
Insulating foam - sprayed-on or as panels attached to the interior walls
Concrete does not have a very good insulating value, and neither does steel. The mixture of the two will add structural stability to what you are building, but it (commonly) will not enhance the concrete's insulating value.
Costs vary by geographic location.
Insulating walls and windows, weatherstripping doors and windows.
some foam for inside walls solid styrofoam sheets along with ty-vek on outside seals so good you have to push the front door to get it closed
depends on where you live, what shops are next to you, for example, if you in sydney, australia, in New South Wales, there is Kmart has cheap posters for walls.
white paint is cheap
Yes, But if you have a cheap adapter then it will be slower.
The R-value of insulating material is an indication of its effectiveness in reducing heat flow through walls, floor, and ceiling of a structure.