Back when I had my `08 M6 convertible, a set of 20" Michelin Pilots would run me about $2100...I did the original plus 2 sets with that car; all on account of tread wear (no flats or tire damage). I replaced 3 run flat tires (Bridgestones) on my first 6 Series car...insurance paid for all three (flats); I think they would have cost me around $400 a-piece. I had 19s" on my `11 7 Series and never had any problems with them.
For what it's worth....
2013 Porsche Carrera 4
2011 Panamera 4S; 2012 Mini CooperS
Prior BMWs: `05 645CiC; `07 650i; `08 M6 convertible; '08 135i; `11 750ix MSport
"You think that's air you're breathing now?"