How much data does turn by turn navigation use?

I have an apple iPhone 5s on AT&T. I only get 3 gigs of data to use a month and I really have to stretch it out due to the fact I have no Wi-Fi at my house.

So I just got my first car and I plan on using apple maps to get me around a lot because I have a terrible sense of direction. Is it going to eat up my data like crazy or not so much?

For example, approximately how much data would be used if I ran turn by turn navigation with apple maps for 30 minutes?

I used my nav system on my phone for 3 hours and used about 30 mb