According to articles on the subject, there is no revenue in users like your wife. But the truly desperate woman blowing 10k on FarmVille credits covers her and 10,000 other 99c purchasers.
Freemium games don't have some secret way of monetizing all users effectively, quite the contrary. They give the product away for free to increase exposure and a small subset of users with piles of disposable income or a high credit limit pay for everyone else, and that frequently results in making more than what they would make if they charged everyone some small fee.
My friend who works at a Zynga competitor claims that they make most of their money from what they call "Whales", a tiny subset of the playing (and paying) population that goes way overboard and spends $10k+. The entire point of getting other people to play is just to get the whales in the door.
Freemium games don't have some secret way of monetizing all users effectively, quite the contrary. They give the product away for free to increase exposure and a small subset of users with piles of disposable income or a high credit limit pay for everyone else, and that frequently results in making more than what they would make if they charged everyone some small fee.