An Iphone4. Cost = $0 for video (already had one). $15 and you can get Harry's Lap Timer, which will allow you to datalog OBD2 information alongside/overlayed on the video using a PLX OBD Wifi (
PLX Kiwi Wifi - Connect your iPhone / iPod Touch to your car's OBD2 OBDII Port.).
example: (not me)
The data can also be exported for analysis.
There is a similar app for Android phones.