The previous answer are wide ranging because there are many variables. If you're doing a simply room addition such as a bedroom, you have electrical involved. Add a bathroom and you've also added plumbing and therefore cost. Be on the side of a hill and need extra support? Add more $. Coastal commission, set backs, special permits - all add more cost. Be sure to get referrals and check references. Anything over $500 requires a general contractors license these days. And, don't forget the cost of permits. Good luck!
Diane Conaway, RE/MAX United, (760) 749-2888
You can get some idea of the cost using this remodeling estimator site...
Here is anothe remodeling cost estimator site to cross check the information.
I agree Jed that many construction wokers are under employed or unemployed. Make sure you use a licensed contractor.