IPad – how much traffic iOS Maps using while in turn-by-turn navigation mode

I'm trying to estimate necessary data plan for a Euro trip, and my main concern is: I'm going to use an iPad mini native Maps app for turn-by-turn navigation being there. Was unsuccessfully trying to find out how much traffic driving let's say from Prague to Berlin (it is a 3-4 hours trip) might use.

I understand I can pre-load/cache some maps for the route while I'm still on wifi at the starting point. But I feel that is very unreliable approach, and it's better to be able to have cellular data plan activated for the entire trip. AT&T (my device is used with AT&T in the U.S.) sells 300MB of international data plan for $60. If let's say I'm considering not buying local SIM with local data plan in Europe, is there a way to predict/estimate how much data plan traffic a 3-4 hours trip through Europe might consume? And how much driving time those 300MB will buy me.