What is the cheapest prepaid data-only plan for using an Android phone as a car GPS tracker? [on hold]

I have an LG Optimus Elite lying around and I got the idea to use it as a GPS tracker in my car (in case it's stolen or I don't know where the wife is). I don't want to add another line to my plan. Ideally, I'd like to find a very cheap low-speed data plan that will likely be prepaid or pay-as-you-go. I will also keep the phone connected to USB in the car so it can charge while it's being driven. I'll put some data-restricting measures in place too so it only sends GPS data over the network.

Any thoughts or ideas on what US companies have a data plan ideal for this kind of use? I've only found suggestions for overseas companies.