The NorCal FasTrak doesn't charge a monthly fee or service charge or anything. It's just pay as you go.
I know some of the SoCal ones charge extra on top of whatever the toll is. It may be worth it to try getting from BayAreaFastrak.com even if you're in SoCal and using it until they kick you off (supposedly based on what % you use in SoCal vs NorCal).