The display is broken by the laptop. The manufacturer wants to have 300 euro for the repair. The part cost 400 is it still worth it? I have now built another one which was cheaper but notice that it is darker I'm totally confused what to do.
It depends on how much you care about the laptop.
For me personally, it wouldn't be worth it.
Have you already checked whether anyone can repair the cheapest?
Otherwise you have to see if you can make friends with the darker screen and if necessary see what can be done in the settings to improve the brightness.
For the 300 euro there are certainly good, new laptops, maybe even the same used ones.
Unfortunately, when deciding on a computer repair, one also has to consider the side effects. If you dispose of the entire part and buy a new notebook, then you may even get cheaper in the short term. But then you would have to set up a new notebook. This concerns something like… Programs, games, pictures, videos, browser history / bookmarks and then other settings. In addition, unfortunately, every device has its quirks. My current notebook e.g. Activates the function keys again and again with every BIOS update (which come automatically). When I press F2 it goes down. I have to press FN + F2 so that I "only" press F2. Can you change in the BIOS that it works like any computer. But as I said with every BIOS update, it is back to factory settings. So far, every notebook I have had something like that.