First answer is not rate per sqft as in the question ... much less even in the ballpark for building costs today.
I have several years experience in remodels/additions in various states and types of construction.
I would agree with second answer for around $200/sqft ...with additional costs for bathroom/fixtures.
Things can be done very cheap($100/sqft) with used items and cheap labor ... but you get what you pay for.
When you go the cheap route you open yourself to many headaches, court battles, incompleted projects, not to mention the problems later on due to taking shortcuts.
My best advice is to get three bids ... and only take the lowest bid if it is with in 10% of the middle bid ... and only then if they have 2 or 3 good references on recent projects. Use your intuition ... not your math. Get every thing agreed to on paper in a plan ... as if it were for some one not having the conversations about all of your detials and options wanted. Agree to payment schedules and completion dates on the contract you sign!