Wells Fargo Home Mortgage
It is going to depend on the contractor you hire, but I have seen add ons as low as $90 per square foot. Get as many referrals as you can from friends, family, coworkers, and so on.
Century 21 Beachside Realtors
http://www.Lehmanhomes.net (Home Search/Foreclosure Search)
It depends on who you hire and how willing they are to give you a break you may be paying anywhere between $180 to $250 per square foot. To know for sure I would interview and talk to 6 to 8 contractors. Everyone will give you a different ideas and that will help you decide on other aspect of you project.
Prudential California Realty
First Team Real Estate