cheapest method to send small (24 bytes) data over long distances (600 miles)

Question

I have a friend who is working on a project where they need to deploy a large number of devices over the midwest. For simplicity let's say these are temperature gauges - they read the current temperature and transmit that information to a server. The server would just need to know what device is reporting what temperature (412X|10c).

These devices will be in forests, near highways, in cities and swamps. All other technology is prototyped and working (ability to read the temperature, the hardware for the device) the open question they have right now is 'what is the cheapest way we can send this information to the primary server'?

I think they'll need to go with a wireless carrier (verizon/sprint/at&t) and use something similar to mobile broadband. Is there really any other option?