You can ask your agent anything you want
Like Matt just mentioned: "We've seen it all over the map!..."
I've seen 6% paid on a $90,000 house sale ($5,400) and I've seen a $10,000 commission paid on a $600,000 home sale .... go figure.
I'm sure in the next 12/15 months we'll be seeing flat fee's across the board, National City is already setting up their structure for it ...
Make sure you have a buyer's agent representing you that has done this a few times (at least)