Alas, this is more common than you think!
The most obvious choices of having apple repair it or purchasing an new logic board are both very expensive-- representing a huge percentage of full replacement cost.
One alternative that has not yet been mentioned is that there are a few people who specialize in repairing logic boards, and do so typically for about $250 to $300. Go to eBay and search "macbook logic board repair". These guys clean them, try to identify what chips have been damaged, replace chips if possible and so forth.
Thanks to eBay's feedback system, you can know that these are not scams and many other people have had success with a particular repair shop.
I sent out a 15" unibody just before Christmas and its repaired and on its way back now.
Lastly-- if repairing is not your thing-- remember that you can still get a surprisingly good amount of cash for your liquid damaged laptop on eBay-- probably more than you think!