Calculating Digital Signage ROI (or kiosk ROI)

I've posted the first part of a Digital Signage ROI article, over at WireSpring's Dynamic Digital Signage and Interactive Kiosk Weblog. While it is slanted towards digital signage, a lot of the base information is certainly applicable to interactive kiosks as well. Here's part of the intro from that article:

"In this, the first in a series of articles covering the basics of calculating digital signage ROI metrics, I'm going to start by introducing a couple of concepts to get you in the right mindset for determining your signage network's return on investment. In subsequent articles, I will try to provide some ideas and methodologies for accumulating and analyzing your data to come up with the magic number."