The price of a motherboard repair merits a new laptop IMO.
Again, it really depends on the computer in question. A liquid damaged macbook motherboard (which is probably one of the worst damages) can be repaired for a few hundred bucks, much less than the cost of a brand new one. You're a lot better off finding an independent repair place (never an Apple store, they are ripoffs) and trying to get it fixed, especially if you can find a no-fix-no-fee place.