I think it's the rare customer who can out-maneuver a builder. The builder wants to bid the project cheap to get the job and then up the profit with change orders, knowing it's extremely likely some of these things will occur:
- The customer will not have thought through every single detail. Something not specified until mid-project costs more than the same thing factored into the initial plans.
- The customer will realize they want something changed in mid-stream.
- The builder will find a flaw in the plans that require something be changed.
Even if you go with a firm fixed price contract instead of cost-plus, you're going to still run into extra costs from change orders.
I think the smart thing is to plan for an extra 15% over your initial price and if you do better, pat yourself on the back.
We never had a custom home built, but I deal with a lot of contracts that end up having to be modified during execution.