It really, really depends. Depends on what you're having done (whether it's a true gut), and where it's being done. It also depends on the level of rehab you're doing--for instance, whether the countertops will be Formica or granite; whether the appliances will be black or stainless steel; whether the flooring will be carpet or hardwood. For example, investors in Baltimore, MD, expect to pay about $70,000 (maybe less, especially if it's for a rental) to nicely finish a gutted shell. They'll buy the shell for $15,000, put $60,000-$70,000 in, and then sell it for $130,000-$160,000. Or they'll rent it for $800-$1,100 a month.
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.