This one is kind of long and a little off color.

A man and a woman get married. On their wedding night the wife charges the husband $20 to make love. He humors her. Every time they make love the wife charges him $20. The husband thinks this is her way of buying new clothes and getting her hair done. After 15 years the husband comes home distraught. He tells his wife he was fired from his job and they are up **** creek without a paddle. The wife smiles and laughs. What's so funny says the husband. All the money you gave me for sex I invested over the years. We will be fine. At this the husband becomes even more distraught. Well what's wrong says the wife. The husband replies If I knew you were doing that I would have given you all my business. :D