Question: What should a room addition usually cost?
Answer: That is almost impossible to answer. You can buy a new car for as low as $6,000 and as high as $150,000. Both will get you from one point to another. A room addition not only includes the known factors, such as the quality of construction, the size of the room and the materials to be used, but also the room location and accessibility.
Identical home improvements to identical houses that may be located side by side could differ considerably in cost based upon the accessibility of area, the condition of the existing property, whether or not walls or fences or trees must be removed, whether the location of the room addition will be on a hillside and many other factors. I recently observed a case in Santa Ana where virtually identical room additions to identical houses in a development had a $3,000 or $4,000 differential based upon these very same factors.
My recommendation is that if you have a clear idea of what it is you want to do, you can determine what the reasonable charges would be by interviewing four or five contractors to do the job. You will find that most of them will be in the same price area for a similar type of work.