I have been having good luck using the Telit GM862 module for a vehicle tracker. Just attach antennas for cell phone and GPS , slide a SIM into the slot, apply power and you have a full functioning cell phone with built in GPS and Python interpreter. These modules are available off the shelf for $140 - Qty: 1 from Janus Remote Communications. AT&T pre-paid GoPhone plans are particularly nice for this use. Buy a $25 card and get a SIM with $25 of value on it. You can then add MEdia net which is AT&T's name for GPRS data:
$5 for 1 meg, $10 for 5 meg, and $20 for unlimited. The money on the SIM expires in 90 days and the media net plans are good for 30 days. If you buy a $100 SIM the money doesn't expire for a full year, so it seems like $8.33/month is the cheapest you can get with no contract. There are new data plan providers like Jasper wireless that will sell you buckets of data you can spread over many SIMs if you have at least a couple dozen units The version of Python running on these modules is 1.5.2 which is 8 years old but all the documentation is still available. I have been able open a socket and send data to my server using UDP packets containing location info as well as the tower ID and received power level. I am going to plot this data and try to determine the location of towers around me