If you are planning a journey, find your route and download turn by turn directions whilst you are on WiFi. When in the car, turn off cellular data. The only problem then is if you veer off course; the map would need to reroute and won't be able to do that without data.
Another option is to use a different app.
You need to think about how often you will use Google Maps. If it's a one off, don't worry about it, seriously. If you are using it all the time then maybe you could look at your data plan and purchase an add on bundle.
Alternatively you could get a dedicated Sat Nav. Though I have to say, Google maps works really well, and I use it whenever I need directions. That said, I have unlimited data on my plan, so I don't notice how much it uses, I do know that, as I said earlier, it is very little.
I hope that goes some way to helping.