Screen motion detection software?

I have a certain task I would like to automate as much as possible.

I need to register changes in a small part of a web page. That is, I need to plot down some data on the page each time a change occur somewhere else on the page. I need to do that for a couple of hours and the the time between the changes varies from ten seconds to several minutes. The information I need to plot down is NOT in a text format.

Now as you can imagine I would like to automate that task as much as possible to cut down the time used for this.

First task is to archive each change so I don´t have to monitor the page in real time for the entire duration. Some surveillance software has motion detection and is triggered by movements above a certain threshold in a specified area of the video and I wonder in there is something similar for movements on the screen?

It could either be a trigger for taking a screen shot of a certain area of the screen if there is any change in another certain area of the screen (or just trigger keyboard shortcut based on the movement since I already have the screen shot part taken care of). In pseudocode: if change in pixel color above threshold X in area Y take screenshot of area Z/send keyboard shortcut. I could do a automator script if only I find a solution to the trigger part (monitor the change in specified area of the screen). I could then go through the screen shots one of the time a note down the necessary data.

Or it could be a video solution: I record the entire duration and automate cutting everything but each ten seconds after a movement happens in a specified area of the video.

Or a hybrid: Save a frame of the video when change occur.

Any suggestions. As cheap as possible of course?