Those are high prices. Here in Britain, One would expect to pay one third to one half of those prices for a typical laptop. Possibly your laptop is special in some way? It is not a model sold in Europe. However, unless a laptop is almost new, the cost of a major repair can be greater than its value. Even for a cheap brand like Dell. I agree that the inverter board may be involved, or else the backlight. I am a little surprised if the LCD has to be replaced completely, unless the backlight is integral and cannot be replaced separately. Maybe a new computer would be a better investment.
How important is the portable aspect? You could buy an external LCD monitor for much less than $700.