Sorry, I've been away for ages but still want to reply if you're still interested.
I think economies of scale and wise choices really helped us. Our home is about 4200 sf. We didn't do fancy roof lines or create a geometrically unique footprint. It's basically an L shaped house with a simple roof. All wet areas (kitchen, wet bar, bathrooms, laundry) are placed so we didn't have to run a lot of extra plumbing or wiring. The stove is on an exterior wall to decrease venting costs. We have granite in all areas, but for spaces that didn't need much (kids baths, laundry) we used remnants. So we didn't have a lot of wasted materials. Also, we didn't waste space in our house. Every inch of it is usable. As a result we were able to finish 3/4 of the attic to create a generous man cave and half bath. The other 1/4 is used for storage and the HVAC unit.
We didn't do any of the work (if only you knew how laughable that is
) and we had no connections. I think we just made smart decisions and had a contractor that really educated us on how to stick to our budget while still getting a nicely finished, well built home.