We don't have lifetime sales figures for the Xbox version, but in 2 months the Xbox version moved about 162,005 copies in the US, whereas the PS2 original moved 2-2.3 million in 2 months in the US.
Granted, 2 months after the PS2 version launched there were about 46 million PS2 consoles in circulation, whereas 2 months after the Xbox version launched there were about 8 million Xboxes in circulation. But that still means the PS2 original sold 2.1-2.47 times as much in a 2 month period. So why wasn't it a smashing hit on Xbox? Is it because it came a year later and by then people just bitched online about Raiden?
VGChartz puts lifetime sales for the Xbox version at 620k btw, but VGChartz isn't reliable