Every time you use the native maps app, iPhone clears the cache and reload data every time you check maps. This can be avoided with MapLocker, a tweak that has recently appeared in Cydia.Maplocker will add a sub-menu in Settings where you can select if you want the map cache is cleared or not. If you choose not to clear cache, next time when you use the same map or same place maps will be loaded from cache itself. That means you don’t have to consume data.
You can download MapLocker for free on Cydia, you’ll find it in the BigBoss repo.