Well, 21" tires are EXPENSIVE. Looking at Tire Rack, the Dunlop 21" tires run over $2,100 for a set of 4, and you have two choices in OE 21" sizes: Pirelli snow's and the Dunlops. The Dunlops are a 240 UTQG, so you'll be buying them much more often.
21" Tire Search Results

Choosing 20" wheels/tires instead gives you a lot more options and more affordable options. 9 tires from $950 to $1,900.
20" Tire Search Results

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Scott
On Order: 2015 F15 X5 35i
Space Gray with Mocha Nappa Interior Design Pkg and fineline Oak trim
ZXD | ZCW | ZPP | 3AG | 456 | 4UB | 4U1 | 5DL | 688
To be Traded: 2010 E70 X5 35d
Alpine White on Black with Dark Bamboo trim
ZAP | ZCW | ZPP | ZPS | ZRC | ZTP | 322 | 328 | 330 | 386 | 4AZ | 4UB | 655 | 6FL | 6NF