AKAIK there is no way for to spoof caller ID on a GV call delivered by XMPP.
However, you could add another PAYG SIP provider in a VGx, that has lower per-minute rates than Callcentric and allows you to specify the outbound caller ID. You would use a prefix to select the alternate provider for a given call.
Possibilties include Anveo at $0.01/min. IMO their quality and reliability is as good as Callcentric's (though their track record is much shorter). It would cost you $10 to try them out.
Voxbeam offers $0.0049 "Standard" and $0.0081 "Premium" routes, with six-second billing (some rural destinations are higher). There are a few technical hassles, but it's not that hard to set up. You get $1 credit at signup, so you can test without making a payment.
I've not tried them, but AlcazarNetworks has a good reputation and depending on the specific numbers you call, you'll probably average between $0.0025 and $0.004 per minute, with six-second billing. You get a 24-hour free trial at signup, limited to $5.
If you are a consistently heavy user, you could consider the "unlimited" (5000 minute cap) services such as VOIPo or Phonepower. Neither permits spoofing, so you would have to port your number there. Unfortunately, they are not as reliable as Callcentric.