Just a couple of points to add should you be inclined to consider them:
(1) The GXM-30 IS the actual XM radio receiver (the Zumo is merely the interface), which also contains an antenna -- this is predominately why it seems a bit pricey;
(2) You CAN purchase a GXM-30 for under $200 (as I did at Comp-U-Plus); and
(3) If you did acquire a GXM-30 you certainly don't have to invoke (via a $3.99/mo. subscription) the NavTraffic component.
Otherwise, if you want your XM integrated conveniently with your Zumo, as Jon says, it can only be done with a GXM-30. Of course you could obtain a separate and autonomous satellite radio -- but regrettably, this is not without it's inherent downsides. OK, the ball's in your court now.