Might be they don't have a replacement board available so they actually have to manually repair your board. You're paying for the labor for whatever they're doing as well as the parts so it can get expensive out of warranty.

Personally I would look on ebay for a motherboard or a unit with a broken screen and have a local shop swap the parts out. Should be cheaper.

That's ridiculous. Should be like a car accident calculation; if the cost of the repair is more than a certain percentage of the entire item replacement, then it should be replaced. Makes no sense to repair it at that point, if the repair cost is too high (like in your case here).

Did they already fix it and now are billing you? Or did they just give you the diagnosis and ask you what you want them to do?

that is classed as BER (Beyond Economic Repair) and is a recognised status within IT for equipment, or at least in professional repair centres