because the two AA batteries last for several days even in cold conditions and can be replaced or recharged easily in the field in the waterproof and ruggedised device with a sunlight readable screen?
Contrast that to the phones we all have which last about 20 minutes outside if it's too cold or too bright, can't be easily seen in direct sunlight, and break with the slightest hint of a drop or something else in the bag with them. There's no comparison for the outdoors. iPhones also have a tendency to not switch on if they've been in the sun or the cold for more than a few minutes. That's not helpful for navigation!
Regardless, there is no reason why the same digital mapping has such contrasting pricing between use cases. nor is there a justification for the extremely high price they are charging.