The LCD panel is ok and a external monitor works as well. I am sure it is not too expensive to repair.
The cost depends on the faulty part…
It seems that this issue is not related to the graphic card issue because the external monitor is ok.
Why you are sure that LCD panel is ok?
Can you see something on the notebook’s display???
If yes then the issue can be related to FL inverter. Such part is not expensive but it’s tricky to replace it and I think the best would be to get in contact with an local ASP.
The authorized service partner could provide all needed info about the costs and how to handle this…