What are the differences in quality of shower cabins?

A small "all-from-one-source surcharge," and then it fits more or less.

Our shower door can serve as a (not representative) example here. We had it offered as an additional service by the developer and also in identical form at the local sanitary wholesaler, including delivery and installation. So, two identical services. The developer was somewhat cheaper. IMHO logical, since their sanitary company was already on site anyway, so travel, delivery, and work preparation are calculated differently than in the case of later shower door installation by an "independent" company commissioned by us. The "all-from-one-source surcharge" can therefore also be less than zero.

I did not research internet prices; that was not relevant for us.

I also believe that the price difference hardly arises from differences in quality. In the 1,300 euros, besides the mentioned points, the measurement and the warranty are probably included.

Although I myself tend to include warranty and guarantee services together with the guarantee. I think manufacturers who are confident in their own product offer it more so than budget manufacturers.

However, a high price alone is of course not a guarantee for good quality. The overall picture must fit.