Not familiar with the Samsung Navigation App!
Generally the GPS doesnt use 3G data unless it is using data for fast synchronisition which should only occur when it gets it first lock - you can do this with your WIFI instead.
I have a Samsung Galaxy Nexus which doesnt has this application - does it have on-board maps as TomTom etc do or is the routing data obtained from the network (WIFI or 3G in the same way as Google Maps does?
Google maps will copy all of the route before you set off so you can use WiFi rather than 3G and the routing will work without needing data. The Samsung app may be the same.
All you need to do is switch off the data connections (both 3G and WiFi) and see if the Samsung app still works - if yes it is obviously fine.
Failing that set up the route with WiFi and then switch 3G and WiFi off - does it still work. It it does then again it should be fine.
Personally I think the 250MB limit on a phone such as this is extremely limiting but obviously this is down your choice / limitations etc.
Ex <n>ildram , been to SKY MAX - 15,225 Download
BE Unlimited - 21,000 Download 1,200 Upload ON THE LINE THAT SKY COULD ONLY PROVIDE 15,255 DOWN AND 800 UP ON!!!,
Moved house, now BE Unlimited 6,500 Down, 1Mb/s up - gutted!
FTTC Cab installation commenced 12th April - expect full 80 / 20 - bye bye BE, hello BT Infinity soon!