mobile data comm

Hi all,

Does anyone know of a cheap mobile long distance (20+ miles) data
communication service that would work in a large city environment?

I'm looking into communication of prognostic data on vehicles.
Basically a bunch of vehicles would be traveling around within a
certain range of a main building. They would each send data back (say
once every 5 min) for processing.

Cost is a main factor. I've looked into cell modems, but I'm not aware
of any service without a substantial recurring cost. (For example $30/
month for 200 vehicles adds up fast). Radio modems can operate in
license free bands, but I'm worried about the amount of noise as well
as the distance that would be achievable in a city.

Any thoughts / relevant experiences would be appreciated.

Thanks

Andy