"Could you possibly..." or "Is there any way..." is precisely the right phrasing. And again, the tone used makes or breaks the deal. Furthermore, explicitly quoting the online price is the wrong approach. Ask if there's any way to get $X (online price minus $20-30/night), without explicitly mentioning that you're comparing to the online price. Don't shove the online price in their face (ie: demanding they meet or beat, which is aggressive posturing). Only refer explicitly to the online price if you can't seem to charm them without doing so.
The long and short of it: you're trying to win over another human being to be on your side. You don't demand that a company or "lowly employee" meet your expectations. Tact for the win!
