My suggestion is to look in the classified ads in the various X5 boards for a set of used wheels with either all-season or snow tires. I looked into this and found some decent deals last year but ended up just buying a winter package from my dealer for @$2600. When you go to turn in your X5 at the end of lease BMW requires that you return it with the correct size run flat tires.
Current BMWs;2013 Carbon Black X5 50i M-Sport PP
2010 Alpine White 135ia M-Sport
2010 R1200GS Adventure 30th Anniversary Edition,2014 S1000R
Past BMWs; 2010 Carbon Black M5, 2007 Graphite K1200GT, 2008 335ia, 2005 545i, 2004 R1150R, 2003.5 M3 Cabrio, 2000 540ia, 1997 M3 sedan, 1997 540ia