We signed a contract in summer 2021 for 120sqm turnkey on a slab. With some extras and KfW 40 Plus equipment, it cost around €375,000 back then. However, with 1% interest and higher equity than you. That means €1,200 per month for 20 years for us, then it's paid off.
We obtained about a dozen offers. From solid prefab houses, to the developer, to some prefab house suppliers. Our house provider had the best price-performance ratio. Most were generally more expensive; many demanded €370,000 even in the standard version and with gas heating.
We also got some offers from craftsmen. But my impression was that due to the workload, they really didn’t want new customers. The prices for the individual trades were sometimes even higher than with our prefab house provider.
With kitchen (€15,000), additional costs (€30,000 to €40,000), garage & carport (steel €12,000), terrace and driveway, we will probably end up around €450,000.
Since then, prices have gone up drastically. Absolutely. I could almost observe this monthly with garage offers. Prices for photovoltaic systems are now beyond good and evil – usually not worthwhile over 20 years.
You will probably have to expect significantly higher costs. Interest will be added as well.