Well, if you have to crank up brightness and contrast just to be able to read the screen, the video on the motherboard is definitely putting out a weakened signal. Replacing the motherboard is definitely not a sure thing, though you could try it. Hopefully, the processor and power supply are not fried to some extent as well.
$30 seems reasonable to get it working again. With a lightening strike that's clearly damaged the PC, I'd also recommend replacing the power supply as well.