The cheapest option is going to be through the Apple store or an Apple authorized service provider. When we get in machines that need out of warranty logic board replacements, the cheapest option in an Apple depot mail in. Even though we are Apple authorized it would cost twice as much for us to order in the board and replace it in the shop as opposed to mail it to the repair depot. Apple makes it like this on purpose.
Of course, the catch is there can't be any accidental damage (liquid or drop damage) to the machine. If the machine has any signs of abuse beyond normal wear and tear the price goes up significantly.