You should treat the favor as a gift, and not expect a quid pro quo. You shouldn't do more than you can afford as a gift for them; you shouldn't spend years of your life, or a large fraction of your salary, or anything of the sort. It's just a gift for a friend or family member.

That way, you aren't left hanging, expecting something from them, and having that poison your relationship. Be very careful about letting some kind of debt or obligation hang over the head of a relationship that you care about.