If it's less than a full gut, many investors think about it on a room-by-room basis. A bathroom goes for about $3,000; a kitchen for about $15,000. A replacement window will cost about $200. Those prices will strike a lot of people as low, but the work's being done economically, using adequate grade materials, and--maybe most important--the investors aren't dithering for weeks about what color of yellow to paint the kitchen.
Most houses I see--liveable but in need of some rehab and updating--usually need $30,000-$40,000 worth of work. Saw one like that today--had the pink enamel bathrooms, butcher-block formica kitchen with old painted wood cabinets, old metal sliding windows. But I digress...
Buy a book or two on how to work with contractors. Learn about draws. Learn about different grades and qualities of materials. Make sure the contractor has the appropriate licenses and insurance. And have a lawyer review the contract before you sign.
Hope that helps.
You may find a range of costs per square foot depending on your home size, type of home, what you want to put back in it, and unfortunately your zip code. Some neighborhoods do get charged more than others it is just a fact of life sometimes, so be sharp. Pattie gave you a good baseline to use for some nice finishes. There are remodelers that will do complete additions for $ 100 per sq. ft. but you get what you pay for.
Get several references, look at their work, get a written contract with completion dates, and most important an insurance certificate.
Have fun with the sheetrock dust.
I would shop around and see, I paid for an extension 145 a sq ft, BUT make sure before you sign any contracts, That there be a start date and a FINSH date, and DO NOT PAY it in FULL unless they had you the proper documentation and all permits closed, I have a horrible experience and had to go to court , I won on the fact my contractor never put a start date or finish date and 1 and a half years later I had to finish it at my expense, also: check the better BBB. and make sure he is licenced