I believe if you configure your APN settings (What are the TCP and APN settings? - BlackBerryFAQ.com
) then you will be charged for whatever data you use.
Google maps is pretty data intensive. You don't necessarily download directions and save them. It is more of a real-time data usage application. If you are going to use Google Maps regularly then I would recommend some sort of data plan, even if it is the generic AT&T PDA data plan.