Its because Lexus has been around longer...  Not many people with a warranty are willing to mod their cars.  Especially in this type of vehicle.  The thing about the M is that right when off warranty cars are becoming available at reasonable prices, the economy tanks.  It will come around...

2011 Infiniti M gets a minor price bump

Effective immediately, the M56 and M56x are up by another $350, with a sticker price of $58,775 and $61,275, respectively. The M37 and M37x received a $250 bump, with a new starting price of $46,500 and $48,650, respectively. We're not exactly sure why the price of the M jumped, but it could have something to do with the fact that sales of the flagship Infiniti are up 45 percent on the year, with August 2010 sales more than doubling compared to the same month in 2009.

http://www.autoblog.com/2010/09/15/2011-infiniti-m-gets-more-of-everything-including-a-price-bump/