Hi Skywarn-NL (interesting name)
THe size of 1 update is about 100bytes. 10 updates, 1 kilobye, 100 updates., 10kilobyte
A full 4 hours driving with a ten second interval comes down to about 100 Kilobyte in traffic. Depending on your provider, some also count "connection control datagrams" as traffic. This is called theft, but they do it anyway :)
So , If you sit in your car 2 hours per day, count on 50K traffic.
One update looks like this, so you can count the bytes yourself :
http://tracker1.reperion.com/webform1.aspx?name=Skywarn-nl&latitude=52.313013&longitude=007.221828&speed=5&msg=*&crs=1&ver=1.52.5&passwd=notstupid&Smap=no&zoom=.005&smsUTC=6/26/2007
A community for and by the people, the ideas are endless, enjoy !