Most of the variation is going to be in finishes-- you could spend anywhere from 50-400/sf just in finishes.
You're really going to have to have some contractors look at the work and give you some bids. If you can get 3 bids, you should have a good idea of where the market is.
That question is not easy to answer because the price changes from place to place. In the Fayetteville NC real estate market where I work prices might be $65—$75 per SF whereas Charlotte NC is about $10—$15 more. Check with local community colleges or county inspections, they may keep up with local prices.
It depends on what kind of house and where you are building it. In the Boston area $200 per square foot will build you a very nice house. That doesn’t include the land.