With Google Maps you can download offline maps. You obviously won't get the traffic updates which make Google Maps so good, but you will get much easier to use sat nav and it won't cost you anything in data.
To be honest Google maps isn't that bad data wise. I use less than 1GB of data a month on it, usually much less, and I use Google Maps on almost every journey, even to places I frequent often. There is so much roadworks going on in and round Wolverhampton that you can use the same route twice a day and find temporary lights and roadworks on the second journey.
Google is so good at picking up on these closures, mainly because it owns Waze and feeds it's traffic reporting into Google Maps too, so you have the best of both worlds without Waze's childish looking set up.