What they will come off the asking price depends on what their asking price is to begin with. And also depends if you are looking at a $50K rig or a $100K rig.
If you google a specific model and floorplan, you will find dealers advertising that same floorplan with prices differing by as much as $20K to $30K. Generally they will also list the "MSRP". So if a dealer is already showing a price that is $25K off their "MSRP", then they are not likely to drop it much farther. If they are advertising only the "MSRP", then they will drop the price significantly. (Note that I put MSRP in parenthesis since that is a questionable number to begin with).
Do a lot of searching for the model and floorplan of interest, find the lowest advertised number, and use that as a bargaining tool.