Don't forget the overheads a general contractor has....insurance, bonding, payroll, rent, etc that they have to maintain. It's always cheaper to be your own general contractor if you have the time and knowledge.....
I'm busy with an addition to my house and garage, but the contractor is my Brother in law and he is doing at zero profit (or at least very little), so it helps to have someone that you know and trust when it comes to building contractors....I've heard all the horror stories!!
I personally think that local municipalities are a little easier and more lax with you if you as the "homeowner" pull your own permits.....my contrctor had a hell of a time with the City getting the permits approved....for stupid stuff too!!!