I don’t understand what this calculation actually tells you, unless each printer is only used once, then thrown away.
Is that not how you use your 3d printers?? 
The calculation is really just to see how much build volume you can get for the initial cost of the printer, not the cost of material over time. Although that would be another good metric to rate them by, as the materials for some of these printers are sold only by the manufacturer like you say.
This is all good stuff to know! I don’t have any direct experience with FDM printers so I wasn’t aware how much maintenance they needed. I was considering the Form 1, but since we will be making mostly rough functional prototypes it may be a bit overkill. I think if one of these was super easy to maintain(or just easy to repair?), that would be the one to shoot for.
I suppose the printer software is another thing to consider. I’ve heard the Makerbot software is easy to use, but I don’t know about any of the others. Has anyone worked with the software for these printers?