I'm not sure how this works ... 1) husband is sleeping around. 2) wife says "pay me for sex" ... why in the world would a husband whose sleeping around on his wife pay for sex with his wife.
You might think that sex with his wife was earlier free, but it was happening only when she wasn't mad at him, which was probably not very often.
So either be on your best behavior and occassionally get sex for free, or pay $6 when you want and almost always get sex. Those men probably already chose the second option just with other women and for higher price.
Might work. Also strangely it might bring couples together because having sex with someone just mechanically improves stability of the relationship.