Affordable providers for building a move-in ready house

I think, however, TE used the term prefabricated house in the sense of a house planned and built from a single source.

In the sense of a turnkey house, I agree with your assumption. But beyond that, I assume that we are dealing here with a believer in the advertising myth that the price at the checkout is the same as in the advertisement.

But I also assume that we are dealing here with a believer in the advertising myth that the price at the checkout is the same as in the advertisement.

... which is already installed in the factory.

There used to be a saying "Who buys cheap buys twice" and the saying is still valid today. When you build with local reputable craftsmen, you usually aren't much more expensive. With the house suppliers, they just build cheaply and sell it at a high price. When I watch how the prefabricated houses are assembled here, it makes me feel differently. Regardless of rain or snow, a deadline is a deadline. One thing already needs to be renovated.

Since I assume that by "ready to move in" you also want the ancillary construction costs included... for 250,000 you won't get more than 80m² even with Town & Country or Heinz von Heiden, and that only turnkey, not ready to move in. Forget it.

120m² with a cheap general contractor who builds standard, costs around 275,000 EUR for the house (there is always some upgrading), plus 50,000 ancillary construction costs. Ready to move in costs another 30,000 EUR on top, plus kitchen. And then you still don’t have any furniture. You probably won’t want to take all the furniture from the rental place with you.

There used to be a saying: "Who buys cheap buys twice." And that saying is still valid today.
(...)

Yes and no. Because those who buy cost-effectively do not buy twice. This bashing of the "cheapskates" is annoying.
On the one hand, the cheapskates largely rely on local craft businesses because they don't have their own bricklayers, electricians, roofers, etc.
On the other hand, you simply get better prices if you order 5,000 heating systems from Vaillant instead of 50. That doesn't make the heating system any worse.

And the local craft business... then on weekends the Eastern Europeans show up ;)
I see that every day in the construction area here. Only the screed layer was somewhat competent in the German language, let's say...

If SIXT orders 20,000 cars from BMW, they hardly pay more than the manufacturing price. Does that make all the cars junk?

Yes and no. Because those who buy cheaply don't buy twice. This bashing of the "cheapies" is annoying.
On the one hand, the cheapies largely rely on local craft businesses because they don't have their own masons, electricians, roofers, etc.
On the other hand, you simply get better prices if you order 5,000 heaters from Vaillant instead of 50. That doesn't make the heater worse.

And the local craft business... then the Eastern Europeans come on the weekend ;)
I see that every day in the construction area here. With us, only the screed layer had limited command of the German language, let's say...

If SIXT orders 20,000 cars from BMW, they hardly pay more than the production price. Does that mean all the cars are junk because of that?

True, that was a strange group then, without any markings on the vehicle, who had to get water for us because they were too dumb to clear the house connection themselves.
And then the trades come from 100 km away and every little thing costs extra if you don't just want 2 sockets in the office.

In any case, I wouldn't do something like that.
We have local providers who work together with craftsmen from the region.