Some say that it is unethical to pay someone for a kidney while others say it is unethical to use a method (a government list) that makes fewer donor organs available.
As always, choosing is refusing.
The debate continues in a recent NY Times article:
http://www.nytimes.com/2009/12/22/health/22essa.html

opportunity cost: the alternative that was sacrificed when making a choice; e.g. if pizza is sacrificed when I eat yogurt for lunch then pizza is my opportunity cost.