AIM On Jornada 720 With Vodafone GPRS Card

I have set up our Jornada family (4 J720s) using D-Link DCF660W WIFI cards. I have AIM version 2.0 running on all 4 machines. Instant messaging works great. I was wondering what the cost implications would be for instant mesaging using the Vodafone pay as you go GPRS card for internet access. This would provide internet access wherever a decent vodafone signal was available. The cards come with 5MB of data transfer. What would the data transfer be for messaging with instant messenger? As you are only charged for data exchanged, in theory I could leave my J720 connected all the time and only be chargesd for the data transfer associated with instant messages. Has anyone experience of doing this

Craig