I honestly don't see anyone other than truckers that put on thousands of miles a month being able to justify spending money each month on an app like that.
The basic version of DriveGain is free, but I also think it does NOT talk to the OBD II port. I think it uses GPS to determine your driving style. I assume that your solution would require a (wireless?) OBD II interface. Here are OBDII->wifi options :
i've got one of these;works with torque and android(running an htc hero), only down side is to update the firmware you need an iphone and their software.
I use Torque - it is really cheap and has got most of features I need. It connects to ECU via ELM327 (bluetooth). I once wanted to create my own fuel computer from a scratch but I realized that l already have got a display, GPS receiver, some computing power in my android-based mobile. I really like that app and I honestly recommend it
I use Garmin Mechanic (free android app) and the garmin EcoRoute HD, and while I agree that the Garmin Ecoroute software on their GPSr's is wanting, the OBDII EcoRoute HD system is fairly accurate, if a little slow...
Updates about once a second.