Cost transparency in prefab house companies - Which providers are these?

It exists. We built a house with Wir Leben and received about a 40-page calculation for each offer. Not only what you chose, but also other options. That way, you could compare at home what, for example, the interior walls as solid wood walls, exposed walls, or timber frame cost, or the towel holder.

But that is the exception. It was certainly after the signature and not before.

No from the beginning long before signature

I was also surprised that most companies do NOT provide a "model" of the planned house on the computer or at least on the "drawing board" before signing the contract... my expectations are obviously too high...

Based on your experience, are there providers you would recommend in terms of transparency/quality?

I fundamentally see it differently: I will certainly not sign a contract if I do not know what "my" house costs: that means without a drawn house, discussed features, and fittings. And yes, there are providers who actually do this as part of my evaluation—and these are probably the providers I place more trust in accordingly (of course also based on the documented experiences of other home builders). In my opinion, when it comes to a house, you are dealing with a cost dimension where it no longer really makes sense to buy a pig in a poke. It is rather the rule than the exception that there may be "adjustments" during the final customization. But anyone who knows from the start what they want (and is approximately aware of the "included" services), calls a spade a spade, and the manufacturer lists the costs accordingly will be able to stay within their self-imposed financial framework. Pure "house sellers" would not even get through my door.