Display issues generally are one of 3 components:
- Display panel itself
- Main Logic Board (issue with the video subsystem)
- Components between the two including the Inverter board
If it’s something like an inverter board (generally no backlight but there’s actually an image on the screen) you’re looking at the cheapest repair there. Main Logic Board or display panel are costly. It’s not that they’re ripping you off, it’s that they weren’t exactly sure which component had gone wonky.
It also sounds like they may not have properly set the expectation of what it could cost vs what you’d like it to cost.
Repairs like this are the #1 reason to buy AppleCare - one repair of a Main Logic Board out of warranty is easily enough to buy AppleCare.