I posted a site with a 50 dollars lap timer. It will give you the basic lap time function.
The cheapeast datalogger is your phone, they sell softwares for it for less than $100, but they do have a limit as the GPS on phones log at 2hz rather than 5 or 10 like most datalogger. The key to datalogger is their software is. The traqmate is great as they have easy to use software, but they can be very expensive. Video obviously is a great thing to have, but moving to datalog is a different ball game. If you can lay video to datalogs, it's even better. Both in terms of looks and how it improves you as a driver.
If you plan to keep tracking, just bite the bullet and spend the $700 for the camera and datalogger. You won't regret it.
Here are things you can do with dataloggers
Shot at 2011-06-04
Shot at 2010-09-14
http://www.youtube.com/watch?v=WdrFteR8hCo