It depends on what you put in the home. For example, granite counter tops (which are in style now here in NW Arkansas) cost a lot more than do formica counters, real wood floors are usually more expensive than laminate, which is usually more expensive than carpet. Metal roofs are more expensive than shingles, etc. I think you get the idea.
If you are planning to build a home or make an addition to your current home, contact various builders and get estimates from them. Decide first what you want, so all of them are bidding on the same thing. Then most builders will add a percentage (this varies but is generally between 10-15%) for his costs/profit for his knowledge and for supervising the work. Some builders will do this for a fixed cost billed on a size basis (i.e. cost per square foot).
One thing to keep in mind is to check references of the builder (ask him for names of past customers and ask him to arrange a visit to their homes so you can see the quality of his work). And get everything in writing.