From the article:
"You can buy 56 Facebook Credits for $9.99. Why that ratio? I have no idea. But ten bucks gets you 56.
You can take those credits to FarmVille and spend 50 of them on 25 units of Farm Cash or 7,500 Farm Coins.
Or, to put it another way, you can take approximately $9.00 worth of Facebook credits and buy $5.00 worth of FarmVille currency."