General Contractor markup for adding on to my house. What %?

[Cali Skier]

Hi,

Questions up front:

What % does a general contractor typically charge on a home build or home add on?

$5k +20% on top of cost a good deal?

We are looking at adding at least a master suite, a garage and blowing out a living room on our current home, so a major addition of about 1200 sq ft with a 600 sq ft garage.

I spoke to a general contractor company today that has a service where he finds all the people, builds the plan and schedule.  I become the project manager, make all the phone calls to the subs, and manage the day to day build with reachback for help to the general contractor if I need it.  For this service is $5,000 up front for design work and 3D virtual walk through so I can approve the final design, and I pay them 20% on top of each check I pay to the subs, I am writing all the checks.  

I get to pick the subs from their list and supposedly they compete with each other or at least I can pick the lowest price.  Anything I pay for has a 20% mark up that goes back to the contractor.  If I ask him to manage and do the phone calls he gets 30%.

I have no problem being the project manager, I do it for a living in a completely different field.

The question are:

$5k +20% on top of cost a good deal?

What % does a general contractor typically charge?

Thanks so much for any guidance