Please find more info here: http://www.stagescycling.com/global/products/sram-x9-gxp/

I won this from a race in France but never used.

RRP: $899 USD (~$1370 NZD)

Crank length: 175mm Stages Cycling Power meter for mountain bike

DESCRIPTION The Sram X9 GXP Stages Power meter as replaces the non-drive crank arm for existing SRAM GXP and BB30 model cranks. The X9 GXP is compatible with X7, X9, X0, XX [166 Q model], and other Truvativ GXP models.

SPECIFICATIONS Accuracy: ± 2% Weight: Only adds 20gr to base crank arm Battery Life: 200+ hours (coin cell: CR2032) Power range (Watts): 0 to 2500 Cadence range (rpm): 20-220 Frame Compatibility: Most frames, see fit guide in our Support section Crank Compatibility: X9 GXP is compatible with X7, X9, X0, XX [166 Q model], and other Truvativ GXP models Device Compatibility: ANT+, Bluetooth Smart Ready Software Compatibiity: Training Peaks, Strava, Garmin Training Center and others