or did I read what you said wrong?
It will be cheaper to use e-balance, Blizzard will take less of a cut with it. Also, you can't store cash in your account, but only e-balance. So you will have to deposit and take the cash out with a small cut each time if you want to use pure cash. Kinda standard model.
Blizzard themselves aren't really sure how the real money auction house will do financially.
Ya I kind of figured they would take less of a cut which in the end will give you more outcome in your auction but there will have to be a huge cut in real money auctions so that e-balance will actually be worth taking instead.