Catalog prices are bait prices, but I don't find the calculation with 300k that far off the mark. Maybe it also depends on where you build.
In the end, we had a turnkey price of about 315,000. But this already included: photovoltaic system, carport, controlled residential ventilation, heat pump, construction water, sample selection, electrical upgrades, floor coverings throughout the house.
Construction incidental costs amounted to just under 15,000. Of that, 3000 was for house connections (water, electricity, fiber optic). 3500 for the energy consultant. Just under 3500 surveying costs. Just under 3500 for wastewater/stormwater connection. The rest are construction electricity, insurance, building notification, etc.
The costs can of course vary greatly; it depends on the condition/development level of the plot and much more. Basically, on what you want and what the conditions are. Maybe these figures help you a bit. Otherwise, I can only advise you to make an Excel spreadsheet and enter your estimates there, and refine the table continuously during the process. I really had mine open every day during the planning and construction phase. It is work that pays off, because only you can calculate what your construction project ultimately costs. It is too individual to rely on other opinions. :)