Hi M-
Tom Tynan used to teach a course about being your own building contractor. It was a very imformative course, and I would recommend it if you want to be your own contractor, or want to determine whether that would be a good idea for you. I don't know whether the course is still offered, but a little internet research should turn it up if it is. There are plenty of traps for the unwary. I found the course helpful, even though I intended to hire a contractor. I understood better what I was getting into.
Many contractors build on a "cost plus 15%" or "cost plus 20%" contract. So on the surface it appears that you would save about 15% on the cost of building the house. A cost plus contract doesn't provide a lot of incentive for the builder to save construction costs, since he/she actually makes more money if the costs are higher. But competition among builders and among subs helps to keep the costs down. Builders who do a lot of repeat business can negotiate better prices with subs than a one-off negotiator (like you), and they know who the quality subs are. In real estate development booms like we have been esperiencing, good subs are in short supply.
There will be an "as built" appraisal for financing purposes and the lender won't lend more than a percentage of the appraisal - another cost constraining factor.
My husband built a house as his own contractor. I asked him if one could save as much as 50% by being your own contractor. His response was "Heavens,no!" He said you could save about 15%. If you had the skills to do some of the work yourself, you could save a little more.
There are a lot of other risks to consider in building your own house. Check out what kind of insurance is available to you when you aren't in the business. Builder's risk insurance may not be available, and many houses have accidents and natural disasters that damage the home before completion. (Our neighbor's home sustained significant fire damage when it was 80% complete.) When I built my house, homeowner's insurance during construction was a good bit more expensive than builder's risk. I don't know the situation now.
Theft of materials and tools from the work site is also very common. Probably every builder has experienced that at some time or another.
And finally, as a contractor you have more risk of liability if someone gets hurt on the job or while on the site. (You have some risk for premises liability as the homeowner, but not as much.)
In summary, although being your own contactor is quite doable, you should do a good bit of research before taking it on. There always seem to be some unpleasant surprises when building a house. Educating yourself can reduce the number.
Good luck!
-D.