For this you need hard facts. The simplest is to point them at software size measures and typical work rates. These are measured in work hours per functional service units. If you have a lot of historical data you could try to compare it with databases. Are there any XP repositories of work rates from real XP projects ?
Use COSMIC FFP on old software estimations
Show typical XP development can be quicker and cheaper
Show square desks are cheaper than the designer ones