Blizzard posted a webpage with all the information about their gold and real money auction house for Diablo 3. And it is very clear that they plan to make serious money with that real money auction house: The transaction fee for equipment is $1, the transaction fee for commodities is 15%. And that is if you want the money to go to your "Battle.net Balance", from which you can only use it to buy other items, or to pay things like a WoW subscription. If you want the money to be transferred to your PayPal account, you will pay another 15%.

Thus selling for example a stack of gold for $10 leaves you with $7 on your PayPal account, and Blizzard with $3 (part of which goes to PayPal, presumably). If you sell a sword for $5, Blizzard gets $1.75 ($1 fix plus 15% of $5) and you get $3.25. Assuming the real money auction house has a lot of activity, Blizzard will quickly make some serious cash with those fees. Some people might end up paying more to Blizzard per month for Diablo 3 than for World of Warcraft.

Think of it as a 30% tax rate on your virtual earnings, if you want to cash them out. Well, before paying *real* taxes on them as well. But that is a different story. Casual AH users are probably better off leaving the money on the Battle.net Balance. But if you fell for one of those "get rich quick" Diablo 3 guides, the only rich people will be Blizzard and the guy who sold you the guide.