See, that's what I disagree with... I don't think an OBDII tracker is going to be easily reprogrammed. Once you get rid of the manufacturer's firmware, the thing isn't going to be able to just talk to the cell network, unless there's a modular one that I don't know about. Those things, while mass produced, are pretty proprietary (except for the ODBII interface of course.) Which model of tracker are you looking at?
As for arduino, you can easily buy arduinos for under $80... there are small versions that will probably do what you want for less than $10. If the more barebones versions don't do what you want, get a raspberry pi and a 4g shield. So yea... cost being a factor, you can get hobby boards at FAR less than you'll get the trackers for, and they're very much more easily customized.
I don't mean to call you into question, but why are you so adamant about using the trackers? Cost isn't a good reason to use one, ease of programming isn't a good reason... The only reason I could think for you wanting to do this is to either bypass some security, or worse... trigger a car bomb?... Like I said, I'm not trying call you into question here, but you won't tell us what you're doing with them, or give a good reason why the tracker is a better option so....