Need much more information. Any estimate given by someone outside your area will be a bit off. Labor and materials differ a bit location to location.
How big will the addition be? If it's second floor, remember you'll probably want to build a new roof, which is pricey on it's own. If it's first floor, you may want a basement, which is also somewhat pricey.
Honestly, I would draw up plans and get a few estimates, then ask on TW about them. But becareful, my dad was in the business for years, and he was forced out of it by undercutters who would do a shitty job to save money. Then down the line the homeowner would have to spend more money to fix the problems.
It's not worth it to save on construction if you'll need to make costly repairs two or three years down the line. Find someone with a good record, and check them out with the BBB.