The base-rate service is $12 per month with no long term commitments (multiple devices have a reduced rate discount).
Usage Rates are billed each time you transmit over the RockSTAR. 1 credit is used per tracking position or per 50 characters of message sent or received. 1 credit is also used if you check your mailbox and there are no messages waiting (A mailbox check). Credits do not expire unless you do not use your account at all for 12 months. Credits may be shared/pooled between all of the devices on your account with a multi-rate package.