It has to d/l the maps so yes. You can save a little by starting your route when under wifi. It doesn't use that much though. I tested it once and I think in an hour it used maybe 10-15M or so? It doesn't use that much. It used so little I didn't bother to remember exactly how much.
You can monitor it too, Settings/General/Usage/Cellular Usage. It updates usage if you go Back to Cellular Usage and then forward again.
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