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It's lower cost than you might expect for removing a vital organ. The most risky part of the operation is the anesthesia, though you'll be out of commission for a few weeks. If your remaining kidney fails you're in trouble but I think many places give you priority for receiving a transplant if that happens.
In return you significantly improve someone's quality of life and life expectancy.
In the case of donations to family or friends: compare to other sacrifices for family or friends. It's positive-sum, so reciprocation makes it worth it in such a context.
In the case of donations to strangers: compare to volunteer work. Probably a pretty juicy signal.