There’s a big difference between spec houses and custom houses. I’m building a custom home in King County, WA right now, and it’s almost impossible to build under $300/sqft. And no, doesn’t include cost of land or land preparation costs.
You can build a custom home for the price of a spec home. However it means sticking to the budget instead of upgrading. You can put a $15 light fixture in the bedroom, or a $50 one - spec homes do the former, custom homes all later - it doesn't seem like much but these all add up. The cost to build a custom home and a spec home is nearly the same if you are careful not to do any upgrades. Those upgrades cost money though - if you are planning to live in the house for many years I'd say they are worth it but be aware of what you are getting into.
In my region, the material costs are just ~15% of the total cost to build. It's almost all labor costs.
That depends on the materials. Installing a $600 light fixture is the same labor as a $40 one. Thus a spec home the material cost tends to be a lot less a % of the total costs compared to a custom house. Though some custom changes require more labor, the effort difference between most is 0, and in a few cases slightly negative (if the material cost wasn't higher all spec homes would get these features)