Well I checked Magic Box for some numbers.
XenoGears didn't show up at all on the Japanese platinum chart, meaning it was under 1 million sold there.
PSX Squaresoft Xenogears 239,000
PS2 Namco Xenosaga Episode 1: Der Wille Zur Macht 245,000
Those are for North America in the Platinum chart "honourable mention".
So it would seem that XenoSaga I is atleast on par with XenoGears? However, given the huge cost to make an RPG of this size compared to the PSOne days, I still don't know if its doing well.
I'm not criticizing the game, just the strange episodic format that Monolith/Namco choose, and its general viability (IMO, it won't finish).