$500.00
While her husband was at work, his best friend stopped by and offered her $500 for sex.

At first, she was repulsed, but then she began to think about how much the money could help them out, and how her husband would never find out anyway. So she agreed, and it was great (so I hear).
When her husband came home from work, he asked if his friend had stopped by.

She got the deer-in-headlights look, but managed to stutter that yes he had, and asked why he wanted to know.

The husband, grateful that he had, explained that he was supposed to drop off the $500 that he owed them!