The previous two answers presumed in-ground termites. If they have invaded the house framing itself, then while perimeter (outside the foundation) treatment might help reduce future infestations, if does nothing about flying termites, or for ones already in the house.
Exterminating them requires tenting of the house and fumigation - a 2-3 day operation for a normal size house, during which time you CANNOT enter the house. Price varies a lot from area to area - hopefully one of the two previous answerers will address tenting fumigation cost. I have paid from $2000 to 5000 about 10 years ago.
if you have flying termite infestation you will likely be in subterranium termite country too, so will need both tenting and perimeter treatment.
Get local quotes. Check Angie's List for reviews. If you are on good terms with a realtor, they might be able to give you a ballpark handle and referral too, because this type of treatment is a common home inspection defect that realtors arrange for on behalf of clients.
Be sure the treatment includes (or have followup done by contractor) "entry denial" - blocking all possible entrances - eaves, roof vents with oversize openings or lacking bug screening, holes around utility penetrations, etc. Also, if you do not have them vacumn up ALL the dead termites and carcasses that can be seen in a home inspection,you may end up having to pay for an unnecessary fumigation when you sell the house, because the inspector will have no way to know if the carcasses and casings are from a past or current infestation.